Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On 08/26/16 at 08:45am, "Zhou, Wenjian/周文剑" wrote: > Hi Baoquan, > > Sorry, I misunderstood it before. > Thanks for your detailed explanation. > > Hi Jon and Baoquan, I'm confused about how to adjust the kdump.txt. > Does the patch set v9 still OK? Yeah, I think it's OK. Let's wait for Jon's feekback. > > -- > Thanks > Zhou > > On 08/24/2016 01:06 PM, Baoquan He wrote: > >On 08/22/16 at 09:14am, "Zhou, Wenjian/周文剑" wrote: > >>On 08/19/2016 11:57 PM, Jonathan Corbet wrote: > >>>On Fri, 19 Aug 2016 08:33:21 +0800 > >>>"Zhou, Wenjian/周文剑" wrote: > >>> > I was also confused by maxcpus and nr_cpus before writing this patch. > I think it is a good choice to describe it in kernel-parameters.txt. > > Then, only two things need to be done I think. > One is move the above description to maxcpus= in kernel-parameters.txt. > And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt. > > How do you think? > >>> > >>>That is not quite what I had in mind, sorry. What I would really like to > >>>see in kernel-parameters.txt is an explanation of how those two parameters > >>>differ - what do they do differently and how should a user choose one over > >>>the other? What we have now offers no guidance in that matter. > >>> > >> > >>I thought about it. I think user may not need this. > >>What user really want to know is how to choose. > >>And it is also not a hard work. If nr_cpus is not supported by the ARCH, > >>use maxcpus. > >>Otherwise, nr_cpus. The reason why maxcpus still exists is nr_cpus can't be > >>supported > >>by some ARCHes. > > > >I think Jon is suggesting that a note can be added into > >kernel-parameter.txt to tell what's the difference between nr_cpus and > >max_cpus. I checked code and discussed within our kdump team, max_cpus > >is used to limit how many 'present' cpus are allowed to be brought up > >during system bootup, while nr_cpus is used to set the upper limit of > >'possible' cpus. E.g on my laptop, there are 4 cpus while 4 hotplug > >cpus, altogether 8 possible cpus. Possible cpus slot is for cpu hot > >plug, means during bootup you want to bring up 4 present cpus, but > >later you could physically hot plug 4 others. Because of attribute of > >some static percpu variables, we need pre-allocate memory for all > >possible cpus though some of them may not be really used if no extra > >cpu physically hot plugged after system bootup. > > > >Hence for kdump kernel, people never want to do a cpu hot plug in there. > >That's why we want to use nr_cpus to limit the number of possible cpu to > >save memory. E.g still on my laptop, if I want to do a kdump, the number > >of possible cpu is still 8, but you may want to use only 1 cpu to dump, > >maybe 2 or 3 for parallel dumping. But you absolutely don't want to set > >nr_cpus=8 in your kdump kernel cmdline, though it doesn't cause failure, > >memory is wasted because of percpu pre-allocation. So specifying nr_cpus=1 > >is much better. While with specifying max_cpus=1, the number of possible > >cpu is still 8. That's the reason. On x86_64 and s390, there's another > >kernel para "possible_cpus=xx" which can be used to set possible cpus for > >cpu hot plug. Only when "possible_cpus=0" is specified, smp is disabled. > >I am not very sure why this is introduced, number of possible cpu is > >decided by the min value of nr_cpus= and possible_cpus=. > > > >nr_cpus and maxcpus might not be very clear to people which are > >described in Documentation/kernel-parameters.txt. > > > >Hi Jon, do you think change as below is OK to you? > > > > > > From 8b940193a29acf0857d4975d77f4b9f48e2d6cb8 Mon Sep 17 00:00:00 2001 > >From: Baoquan He > >Date: Wed, 24 Aug 2016 11:14:34 +0800 > >Subject: [PATCH] docs: kernel-parameter : Improve the description of nr_cpus > > and maxcpus > > > > From the old description people still can't get what's the exact > >difference between nr_cpus and maxcpus. Especially in kdump kernel > >nr_cpus is always suggested if it's implemented in the ARCH. The > >reason is nr_cpus is used to limit the max number of possible cpu > >in system, the sum of already plugged cpus and hot plug cpus can't > >exceed its value. However maxcpus is used to limit how many cpus > >are allowed to be brought up during bootup. > > > >Signed-off-by: Baoquan He > >--- > > Documentation/kernel-parameters.txt | 20 +--- > > 1 file changed, 13 insertions(+), 7 deletions(-) > > > >diff --git a/Documentation/kernel-parameters.txt > >b/Documentation/kernel-parameters.txt > >index 46c030a..25d3b36 100644 > >--- a/Documentation/kernel-parameters.txt > >+++ b/Documentation/kernel-parameters.txt > >@@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be > >entirely omitted. > > than or equal to this physical address is ignored. > > > > maxcpus=[SMP] Maximum number of processors that an SMP kernel > >-should make use of. maxcpus=n : n >= 0
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On 08/25/16 at 01:10pm, Jonathan Corbet wrote: > On Wed, 24 Aug 2016 13:06:45 +0800 > Baoquan He wrote: > > > Hi Jon, do you think change as below is OK to you? > > So nr_cpus is the maximum value, and maxcpus is the current number. > Figures. No wonder the documentation is confusing... Yes. Thanks for reviewing this patchset. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
Hi Baoquan, Sorry, I misunderstood it before. Thanks for your detailed explanation. Hi Jon and Baoquan, I'm confused about how to adjust the kdump.txt. Does the patch set v9 still OK? -- Thanks Zhou On 08/24/2016 01:06 PM, Baoquan He wrote: On 08/22/16 at 09:14am, "Zhou, Wenjian/周文剑" wrote: On 08/19/2016 11:57 PM, Jonathan Corbet wrote: On Fri, 19 Aug 2016 08:33:21 +0800 "Zhou, Wenjian/周文剑" wrote: I was also confused by maxcpus and nr_cpus before writing this patch. I think it is a good choice to describe it in kernel-parameters.txt. Then, only two things need to be done I think. One is move the above description to maxcpus= in kernel-parameters.txt. And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt. How do you think? That is not quite what I had in mind, sorry. What I would really like to see in kernel-parameters.txt is an explanation of how those two parameters differ - what do they do differently and how should a user choose one over the other? What we have now offers no guidance in that matter. I thought about it. I think user may not need this. What user really want to know is how to choose. And it is also not a hard work. If nr_cpus is not supported by the ARCH, use maxcpus. Otherwise, nr_cpus. The reason why maxcpus still exists is nr_cpus can't be supported by some ARCHes. I think Jon is suggesting that a note can be added into kernel-parameter.txt to tell what's the difference between nr_cpus and max_cpus. I checked code and discussed within our kdump team, max_cpus is used to limit how many 'present' cpus are allowed to be brought up during system bootup, while nr_cpus is used to set the upper limit of 'possible' cpus. E.g on my laptop, there are 4 cpus while 4 hotplug cpus, altogether 8 possible cpus. Possible cpus slot is for cpu hot plug, means during bootup you want to bring up 4 present cpus, but later you could physically hot plug 4 others. Because of attribute of some static percpu variables, we need pre-allocate memory for all possible cpus though some of them may not be really used if no extra cpu physically hot plugged after system bootup. Hence for kdump kernel, people never want to do a cpu hot plug in there. That's why we want to use nr_cpus to limit the number of possible cpu to save memory. E.g still on my laptop, if I want to do a kdump, the number of possible cpu is still 8, but you may want to use only 1 cpu to dump, maybe 2 or 3 for parallel dumping. But you absolutely don't want to set nr_cpus=8 in your kdump kernel cmdline, though it doesn't cause failure, memory is wasted because of percpu pre-allocation. So specifying nr_cpus=1 is much better. While with specifying max_cpus=1, the number of possible cpu is still 8. That's the reason. On x86_64 and s390, there's another kernel para "possible_cpus=xx" which can be used to set possible cpus for cpu hot plug. Only when "possible_cpus=0" is specified, smp is disabled. I am not very sure why this is introduced, number of possible cpu is decided by the min value of nr_cpus= and possible_cpus=. nr_cpus and maxcpus might not be very clear to people which are described in Documentation/kernel-parameters.txt. Hi Jon, do you think change as below is OK to you? From 8b940193a29acf0857d4975d77f4b9f48e2d6cb8 Mon Sep 17 00:00:00 2001 From: Baoquan He Date: Wed, 24 Aug 2016 11:14:34 +0800 Subject: [PATCH] docs: kernel-parameter : Improve the description of nr_cpus and maxcpus From the old description people still can't get what's the exact difference between nr_cpus and maxcpus. Especially in kdump kernel nr_cpus is always suggested if it's implemented in the ARCH. The reason is nr_cpus is used to limit the max number of possible cpu in system, the sum of already plugged cpus and hot plug cpus can't exceed its value. However maxcpus is used to limit how many cpus are allowed to be brought up during bootup. Signed-off-by: Baoquan He --- Documentation/kernel-parameters.txt | 20 +--- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 46c030a..25d3b36 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted. than or equal to this physical address is ignored. maxcpus=[SMP] Maximum number of processors that an SMP kernel - should make use of. maxcpus=n : n >= 0 limits the - kernel to using 'n' processors. n=0 is a special case, - it is equivalent to "nosmp", which also disables - the IO APIC. + will bring up during bootup. maxcpus=n : n >= 0 limits + the kernel to bring up 'n' processors. Surely after + bootup you can bring up the other plugged cpu by executing
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On Wed, 24 Aug 2016 13:06:45 +0800 Baoquan He wrote: > Hi Jon, do you think change as below is OK to you? So nr_cpus is the maximum value, and maxcpus is the current number. Figures. No wonder the documentation is confusing... Anyway, this is much more along the lines of what I was hoping for, yes; I'll go ahead and apply it. Many thanks for humoring me on this! jon -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On 08/22/16 at 09:14am, "Zhou, Wenjian/周文剑" wrote: > On 08/19/2016 11:57 PM, Jonathan Corbet wrote: > >On Fri, 19 Aug 2016 08:33:21 +0800 > >"Zhou, Wenjian/周文剑" wrote: > > > >>I was also confused by maxcpus and nr_cpus before writing this patch. > >>I think it is a good choice to describe it in kernel-parameters.txt. > >> > >>Then, only two things need to be done I think. > >>One is move the above description to maxcpus= in kernel-parameters.txt. > >>And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt. > >> > >>How do you think? > > > >That is not quite what I had in mind, sorry. What I would really like to > >see in kernel-parameters.txt is an explanation of how those two parameters > >differ - what do they do differently and how should a user choose one over > >the other? What we have now offers no guidance in that matter. > > > > I thought about it. I think user may not need this. > What user really want to know is how to choose. > And it is also not a hard work. If nr_cpus is not supported by the ARCH, use > maxcpus. > Otherwise, nr_cpus. The reason why maxcpus still exists is nr_cpus can't be > supported > by some ARCHes. I think Jon is suggesting that a note can be added into kernel-parameter.txt to tell what's the difference between nr_cpus and max_cpus. I checked code and discussed within our kdump team, max_cpus is used to limit how many 'present' cpus are allowed to be brought up during system bootup, while nr_cpus is used to set the upper limit of 'possible' cpus. E.g on my laptop, there are 4 cpus while 4 hotplug cpus, altogether 8 possible cpus. Possible cpus slot is for cpu hot plug, means during bootup you want to bring up 4 present cpus, but later you could physically hot plug 4 others. Because of attribute of some static percpu variables, we need pre-allocate memory for all possible cpus though some of them may not be really used if no extra cpu physically hot plugged after system bootup. Hence for kdump kernel, people never want to do a cpu hot plug in there. That's why we want to use nr_cpus to limit the number of possible cpu to save memory. E.g still on my laptop, if I want to do a kdump, the number of possible cpu is still 8, but you may want to use only 1 cpu to dump, maybe 2 or 3 for parallel dumping. But you absolutely don't want to set nr_cpus=8 in your kdump kernel cmdline, though it doesn't cause failure, memory is wasted because of percpu pre-allocation. So specifying nr_cpus=1 is much better. While with specifying max_cpus=1, the number of possible cpu is still 8. That's the reason. On x86_64 and s390, there's another kernel para "possible_cpus=xx" which can be used to set possible cpus for cpu hot plug. Only when "possible_cpus=0" is specified, smp is disabled. I am not very sure why this is introduced, number of possible cpu is decided by the min value of nr_cpus= and possible_cpus=. nr_cpus and maxcpus might not be very clear to people which are described in Documentation/kernel-parameters.txt. Hi Jon, do you think change as below is OK to you? >From 8b940193a29acf0857d4975d77f4b9f48e2d6cb8 Mon Sep 17 00:00:00 2001 From: Baoquan He Date: Wed, 24 Aug 2016 11:14:34 +0800 Subject: [PATCH] docs: kernel-parameter : Improve the description of nr_cpus and maxcpus >From the old description people still can't get what's the exact difference between nr_cpus and maxcpus. Especially in kdump kernel nr_cpus is always suggested if it's implemented in the ARCH. The reason is nr_cpus is used to limit the max number of possible cpu in system, the sum of already plugged cpus and hot plug cpus can't exceed its value. However maxcpus is used to limit how many cpus are allowed to be brought up during bootup. Signed-off-by: Baoquan He --- Documentation/kernel-parameters.txt | 20 +--- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 46c030a..25d3b36 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted. than or equal to this physical address is ignored. maxcpus=[SMP] Maximum number of processors that an SMP kernel - should make use of. maxcpus=n : n >= 0 limits the - kernel to using 'n' processors. n=0 is a special case, - it is equivalent to "nosmp", which also disables - the IO APIC. + will bring up during bootup. maxcpus=n : n >= 0 limits + the kernel to bring up 'n' processors. Surely after + bootup you can bring up the other plugged cpu by executing + "echo 1 > /sys/devices/system/cpu/cpuX/online". So maxcpus + only takes effect during system bootup. + While
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On 08/19/2016 11:57 PM, Jonathan Corbet wrote: On Fri, 19 Aug 2016 08:33:21 +0800 "Zhou, Wenjian/周文剑" wrote: I was also confused by maxcpus and nr_cpus before writing this patch. I think it is a good choice to describe it in kernel-parameters.txt. Then, only two things need to be done I think. One is move the above description to maxcpus= in kernel-parameters.txt. And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt. How do you think? That is not quite what I had in mind, sorry. What I would really like to see in kernel-parameters.txt is an explanation of how those two parameters differ - what do they do differently and how should a user choose one over the other? What we have now offers no guidance in that matter. I thought about it. I think user may not need this. What user really want to know is how to choose. And it is also not a hard work. If nr_cpus is not supported by the ARCH, use maxcpus. Otherwise, nr_cpus. The reason why maxcpus still exists is nr_cpus can't be supported by some ARCHes. I think it may be why the author didn't write too much description of it. I suspect that may be a bit more than you had signed up to do. As an intermediate step, how about this: rather than tacking on those lines in kdump.txt, rewrite that paragraph to simply say what the reader should use. If nr_cpus is good for everybody, just say that, but your previous patch suggests that the situation isn't quite that simple? Actually, if nr_cpus always usable, there won't be these discussions. -- Thanks Zhou -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On Fri, 19 Aug 2016 08:33:21 +0800 "Zhou, Wenjian/周文剑" wrote: > I was also confused by maxcpus and nr_cpus before writing this patch. > I think it is a good choice to describe it in kernel-parameters.txt. > > Then, only two things need to be done I think. > One is move the above description to maxcpus= in kernel-parameters.txt. > And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt. > > How do you think? That is not quite what I had in mind, sorry. What I would really like to see in kernel-parameters.txt is an explanation of how those two parameters differ - what do they do differently and how should a user choose one over the other? What we have now offers no guidance in that matter. I suspect that may be a bit more than you had signed up to do. As an intermediate step, how about this: rather than tacking on those lines in kdump.txt, rewrite that paragraph to simply say what the reader should use. If nr_cpus is good for everybody, just say that, but your previous patch suggests that the situation isn't quite that simple? Thanks, jon -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
On Thu, 18 Aug 2016 11:11:46 +0800 Zhou Wenjian wrote: Thank you for working to improve the documentation! > * We generally don' have to bring up a SMP kernel just to capture the >dump. Hence generally it is useful either to build a UP dump-capture >kernel or specify maxcpus=1 option while loading dump-capture kernel. > + Note, though maxcpus always works, you should replace it by nr_cpus to > + save memory if supported by the current ARCH, such as x86. So, IMHO, this seems like the wrong place for this. I've just spent a bit of time staring at kernel-parameters.txt, and there is no way for a clueless user like me to know what the difference is between maxcpus= and nr_cpus= would be. A far better patch would be to update the documentation there to make that clear. Any chance you would be willing to do that? Then, rather than tacking an "ignore what you just read" note into kdump.txt, it could maybe be rewritten to simply say what users should do? Thanks, jon -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
Hi Jonathan, Thanks for your reply. On 08/19/2016 01:18 AM, Jonathan Corbet wrote: On Thu, 18 Aug 2016 11:11:46 +0800 Zhou Wenjian wrote: Thank you for working to improve the documentation! * We generally don' have to bring up a SMP kernel just to capture the dump. Hence generally it is useful either to build a UP dump-capture kernel or specify maxcpus=1 option while loading dump-capture kernel. + Note, though maxcpus always works, you should replace it by nr_cpus to + save memory if supported by the current ARCH, such as x86. So, IMHO, this seems like the wrong place for this. I've just spent a bit of time staring at kernel-parameters.txt, and there is no way for a clueless user like me to know what the difference is between maxcpus= and nr_cpus= would be. A far better patch would be to update the documentation there to make that clear. Any chance you would be willing to do that? Then, rather than tacking an "ignore what you just read" note into kdump.txt, it could maybe be rewritten to simply say what users should do? I was also confused by maxcpus and nr_cpus before writing this patch. I think it is a good choice to describe it in kernel-parameters.txt. Then, only two things need to be done I think. One is move the above description to maxcpus= in kernel-parameters.txt. And the other is replace maxcpus with maxcpus/nr_cpus in kdump.txt. How do you think? -- Thanks Zhou -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
[PATCH v9 1/2] Documentation: kdump: remind user of nr_cpus
nr_cpus can help to save memory. So we should remind user of it. Signed-off-by: Zhou Wenjian Acked-by: Baoquan He Acked-by: Xunlei Pang --- Documentation/kdump/kdump.txt | 2 ++ 1 file changed, 2 insertions(+) diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt index 88ff63d..d900080 100644 --- a/Documentation/kdump/kdump.txt +++ b/Documentation/kdump/kdump.txt @@ -393,6 +393,8 @@ Notes on loading the dump-capture kernel: * We generally don' have to bring up a SMP kernel just to capture the dump. Hence generally it is useful either to build a UP dump-capture kernel or specify maxcpus=1 option while loading dump-capture kernel. + Note, though maxcpus always works, you should replace it by nr_cpus to + save memory if supported by the current ARCH, such as x86. * For s390x there are two kdump modes: If a ELF header is specified with the elfcorehdr= kernel parameter, it is used by the kdump kernel as it -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
