Re: [Devel] [vzlin-dev] [PATCH vz7] fuse: relax i_mutex coverage in fuse_fsync
Maxim Patlasov writes: > Alexey, > > > You're right. And while composing the patch I well understood that it's > possible to rework fuse_sync_writes() using a counter instead of > negative bias. But the problem with flush_mtime still exists anyway. > Think about it: we firstly acquire local mtime from local inode, then > fill and submit mtime-update-request. Since then, we don't know when > exactly fuse daemon will apply that new mtime to its metadata > structures. If another mtime-update is generated in-between (e.g. "touch > -d file", or even simplier -- just a single direct write > implicitly updating mtime), we wouldn't know which of those two > mtime-update-requests are processed by fused first. That comes from a > general FUSE protocol limitation: when kernel fuse queues request A, > then request B, it cannot be sure if they will be processed by userspace > as or . > > > The big advantage of the patch I sent is that it's very simple, > straightforward and presumably will remove 99% of contention between > fsync and io_submit (assuming we spend most of time waiting for > userspace ACK for FUSE_FSYNC request. There are actually three questions > to answer: > > > 1) Do we really must honor a crazy app who mixes a lot of fsyncs with a > lot of io_submits? The goal of fsync is to ensure that some state is > actually went to platters. An app who races io_submit-s with fsync-s > actually doesn't care which state will come to platters. I'm not sure > that it's reasonable to work very hard to achieve the best possible > performance for such a marginal app. Obiously any filesystem behave like this. Task A(mail-server) may perform write/fsync, task B(mysql) do a lot of io_submit-s All that io may happens in parallel, fs guarantee only that metadata will be serialized. So all that concurent IO flowa to blockdevice which does no have i_mutex so all IO indeed happen concurrently. But when we dealt with fs-in-file (loop/ploop/qemu-nbd) we face i_mutex on file. For general filesystem (xfs/ext4) we grab i_mutex only on write path, fsync is lockless. But int case of fuse we artificially introduce i_mutex inside fsync which basically kill concurrency for upper FS. As result we have SMP scalability as we have in Linux-v2.2 with single mutex in VFS. BTW: I'm wondering why do we care about mtime at all. for fs-in-file we can relax that, for example flush mtime only on fsync, and not for fdatasync. > > > 2) Will the patch (in the form I sent it) break something? I think no. > If you know some usecase that can be broken, let's discuss it in more > details. > > > 3) Should we expect some noticeable (or significant) improvement in > performance comparing fuse_fsync with no locking at all vs. the locking > we have with that patch applied? I tend to think that the answer is "no" > because handling FUSE_FSYNC is notoriously heavy-weight operation. If > you disagree, let's firstly measure that difference in performance > (simply commenting out lock/unlock(i_mutex) in fuse_fsync) and then > start to think if it's really worthy to fully re-work locking scheme to > preserve flush_mtime correctness w/o i_mutex. > > > Thanks, > > Maxim > > > On 11/30/2016 05:09 AM, Alexey Kuznetsov wrote: >> Sorry, missed that pair fuse_set_nowrite/fuse_release_writes >> can be done only under i_mutex. >> >> IMHO it is only due to bad implementation. >> If fuse_set_nowrite would be done with separate >> count instead of adding negative bias, it would >> be possible. >> >> >> On Wed, Nov 30, 2016 at 3:47 PM, Alexey Kuznetsov >> wrote: >>> Hello! >>> >>> I do not think you got it right. >>> >>> i_mutex in fsync is not about some atomicity, >>> it is about stopping data feed while fsync is executed >>> to prevent livelock. >>> >>> I cannot tell anything about mtime update, it is just some voodoo >>> magic for me. >>> >>> What's about fsync semantics, I see two different ways: >>> >>> A. >>> >>> 1. Remove useless write_inode_now. Its work is done >>> by filemap_write_and_wait_range(), there is no need to repeat it >>> under mutex. >>> 2. move mutex_lock _after_ fuse_sync_writes(), which is essentially >>> fuse continuation forfilemap_write_and_wait_range(). >>> 3. i_mutex is preserved only around fsync call. >>> >>> B. >>> 1. Remove write_inode_now as well. >>> 2. Remove i_mutex _completely_. (No idea about mtime voodo though) >>> 2. Replace fuse_sync_writes() with fuse_set_nowrite() >>> and add release after call to FSYNC. >>> >>> Both prevent livelock. B is obviosly optimal. >>> >>> But A preserves historic fuse protocol semantics. >>> F.e. I have no idea would user space survive truncate >>> racing with fsync. pstorage should survice, though this >>> path was never tested. >>> >>> >>> >>> >>> >>> On Wed, Nov 30, 2016 at 4:02 AM, Maxim Patlasov >>> wrote: fuse_fsync_common() does need i_mutex for fuse_sync_writes() and fuse_flush_mtime(). But when those operations are done, it's actual
[Devel] [PATCH RHEL7 COMMIT] ms/KVM: x86: expose MSR_TSC_AUX to userspace
The commit is pushed to "branch-rh7-3.10.0-327.36.1.vz7.20.x-ovz" and will
appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-327.36.1.vz7.20.8
-->
commit 37f023b9287ec994f33a676f280ebd26816b2dfa
Author: Paolo Bonzini
Date: Thu Dec 1 13:51:12 2016 +0400
ms/KVM: x86: expose MSR_TSC_AUX to userspace
If we do not do this, it is not properly saved and restored across
migration. Windows notices due to its self-protection mechanisms,
and is very upset about it (blue screen of death).
Cc: Radim Krcmar
Cc: sta...@vger.kernel.org
Signed-off-by: Paolo Bonzini
ms commit: 9dbe6cf ("KVM: x86: expose MSR_TSC_AUX to userspace")
https://jira.sw.ru/browse/PSBM-55531
Signed-off-by: Evgeny Yakovlev
---
arch/x86/kvm/x86.c | 11 ++-
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4b2754b..78ea28c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -953,7 +953,7 @@ static u32 msrs_to_save[] = {
MSR_CSTAR, MSR_KERNEL_GS_BASE, MSR_SYSCALL_MASK, MSR_LSTAR,
#endif
MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
- MSR_IA32_FEATURE_CONTROL, MSR_IA32_BNDCFGS
+ MSR_IA32_FEATURE_CONTROL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
};
static unsigned num_msrs_to_save;
@@ -4148,16 +4148,17 @@ static void kvm_init_msr_list(void)
/*
* Even MSRs that are valid in the host may not be exposed
-* to the guests in some cases. We could work around this
-* in VMX with the generic MSR save/load machinery, but it
-* is not really worthwhile since it will really only
-* happen with nested virtualization.
+* to the guests in some cases.
*/
switch (msrs_to_save[i]) {
case MSR_IA32_BNDCFGS:
if (!kvm_x86_ops->mpx_supported())
continue;
break;
+ case MSR_TSC_AUX:
+ if (!kvm_x86_ops->rdtscp_supported())
+ continue;
+ break;
default:
break;
}
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Re: [Devel] [vzlin-dev] [PATCH vz7] fuse: relax i_mutex coverage in fuse_fsync
On 12/01/2016 12:06 AM, Dmitry Monakhov wrote:
Maxim Patlasov writes:
Alexey,
You're right. And while composing the patch I well understood that it's
possible to rework fuse_sync_writes() using a counter instead of
negative bias. But the problem with flush_mtime still exists anyway.
Think about it: we firstly acquire local mtime from local inode, then
fill and submit mtime-update-request. Since then, we don't know when
exactly fuse daemon will apply that new mtime to its metadata
structures. If another mtime-update is generated in-between (e.g. "touch
-d file", or even simplier -- just a single direct write
implicitly updating mtime), we wouldn't know which of those two
mtime-update-requests are processed by fused first. That comes from a
general FUSE protocol limitation: when kernel fuse queues request A,
then request B, it cannot be sure if they will be processed by userspace
as or .
The big advantage of the patch I sent is that it's very simple,
straightforward and presumably will remove 99% of contention between
fsync and io_submit (assuming we spend most of time waiting for
userspace ACK for FUSE_FSYNC request. There are actually three questions
to answer:
1) Do we really must honor a crazy app who mixes a lot of fsyncs with a
lot of io_submits? The goal of fsync is to ensure that some state is
actually went to platters. An app who races io_submit-s with fsync-s
actually doesn't care which state will come to platters. I'm not sure
that it's reasonable to work very hard to achieve the best possible
performance for such a marginal app.
Obiously any filesystem behave like this.
Task A(mail-server) may perform write/fsync, task B(mysql) do a lot of
io_submit-s
All that io may happens in parallel, fs guarantee only that metadata
will be serialized. So all that concurent IO flowa to blockdevice which
does no have i_mutex so all IO indeed happen concurrently.
Looks as you're comparing an app doing POSIX open/read/write/fsync/close
with fs doing submit_bio. This is a stretch. But OK, there is a
similarity. But I don't think this rather vague similarity proves something.
But when we dealt with fs-in-file (loop/ploop/qemu-nbd) we face i_mutex
on file.
This really makes sense. If an app inside a VM loops over ordinary
direct writes, while another app (in the same VM) does fsync, it's not
fair to suspend the first app for long while just because fuse holds
i_mutex for long somewhere deep in fuse_fsync.
For general filesystem (xfs/ext4)
What makes xfs/ext4 general? They are *local* fs having almost direct
access to underlying HDD. Have you explored how CEPH implements fsync?
we grab i_mutex only on write
path, fsync is lockless.
What about the following part of ext4_sync_file:
if (!journal) {
ret = generic_file_fsync(file, start, end, datasync);
? Is it lockless?
But int case of fuse we artificially introduce
i_mutex inside fsync
exactly the same as generic_file_fsync()
which basically kill concurrency for upper FS.
As result we have SMP scalability as we have in Linux-v2.2 with single
mutex in VFS.
You can't derive "kill concurrency" from "introduce i_mutex" as simple
as that. It really does matter for how long the mutex is held and how
often it contends with writes.
BTW: I'm wondering why do we care about mtime at all. for fs-in-file
we can relax that, for example flush mtime only on fsync, and not for fdatasync.
Good catch, makes sense!
2) Will the patch (in the form I sent it) break something? I think no.
If you know some usecase that can be broken, let's discuss it in more
details.
3) Should we expect some noticeable (or significant) improvement in
performance comparing fuse_fsync with no locking at all vs. the locking
we have with that patch applied? I tend to think that the answer is "no"
because handling FUSE_FSYNC is notoriously heavy-weight operation. If
you disagree, let's firstly measure that difference in performance
(simply commenting out lock/unlock(i_mutex) in fuse_fsync) and then
start to think if it's really worthy to fully re-work locking scheme to
preserve flush_mtime correctness w/o i_mutex.
Thanks,
Maxim
On 11/30/2016 05:09 AM, Alexey Kuznetsov wrote:
Sorry, missed that pair fuse_set_nowrite/fuse_release_writes
can be done only under i_mutex.
IMHO it is only due to bad implementation.
If fuse_set_nowrite would be done with separate
count instead of adding negative bias, it would
be possible.
On Wed, Nov 30, 2016 at 3:47 PM, Alexey Kuznetsov wrote:
Hello!
I do not think you got it right.
i_mutex in fsync is not about some atomicity,
it is about stopping data feed while fsync is executed
to prevent livelock.
I cannot tell anything about mtime update, it is just some voodoo
magic for me.
What's about fsync semantics, I see two different ways:
A.
1. Remove useless write_inode_now. Its work is done
by filemap_write_and_wait_range(), there is no need to repeat it
under mutex.
2. move mutex_lock _afte
Re: [Devel] [vzlin-dev] [PATCH vz7] fuse: relax i_mutex coverage in fuse_fsync
On 12/01/2016 10:09 PM, Maxim Patlasov wrote: > On 12/01/2016 12:06 AM, Dmitry Monakhov wrote: > >> Maxim Patlasov writes: >> >>> Alexey, >>> >>> >>> You're right. And while composing the patch I well understood that it's >>> possible to rework fuse_sync_writes() using a counter instead of >>> negative bias. But the problem with flush_mtime still exists anyway. >>> Think about it: we firstly acquire local mtime from local inode, then >>> fill and submit mtime-update-request. Since then, we don't know when >>> exactly fuse daemon will apply that new mtime to its metadata >>> structures. If another mtime-update is generated in-between (e.g. >>> "touch >>> -d file", or even simplier -- just a single direct write >>> implicitly updating mtime), we wouldn't know which of those two >>> mtime-update-requests are processed by fused first. That comes from a >>> general FUSE protocol limitation: when kernel fuse queues request A, >>> then request B, it cannot be sure if they will be processed by >>> userspace >>> as or . >>> >>> >>> The big advantage of the patch I sent is that it's very simple, >>> straightforward and presumably will remove 99% of contention between >>> fsync and io_submit (assuming we spend most of time waiting for >>> userspace ACK for FUSE_FSYNC request. There are actually three >>> questions >>> to answer: >>> >>> 1) Do we really must honor a crazy app who mixes a lot of fsyncs with a >>> lot of io_submits? The goal of fsync is to ensure that some state is >>> actually went to platters. An app who races io_submit-s with fsync-s >>> actually doesn't care which state will come to platters. I'm not sure >>> that it's reasonable to work very hard to achieve the best possible >>> performance for such a marginal app. >> Obiously any filesystem behave like this. >> Task A(mail-server) may perform write/fsync, task B(mysql) do a lot >> of io_submit-s >> All that io may happens in parallel, fs guarantee only that metadata >> will be serialized. So all that concurent IO flowa to blockdevice which >> does no have i_mutex so all IO indeed happen concurrently. > > Looks as you're comparing an app doing POSIX > open/read/write/fsync/close with fs doing submit_bio. This is a > stretch. But OK, there is a similarity. But I don't think this rather > vague similarity proves something. we are speaking about VM process, which essentially re-submits IO from the guest to host like above. For sure QEMU and VM_app have this IO pattern. Thus this pattern MUST be optimized as this is one of our main loads. That is why I think that this case is not marginal and important. Den ___ Devel mailing list Devel@openvz.org https://lists.openvz.org/mailman/listinfo/devel
Re: [Devel] [vzlin-dev] [PATCH vz7] fuse: relax i_mutex coverage in fuse_fsync
On 12/01/2016 11:27 AM, Denis V. Lunev wrote: On 12/01/2016 10:09 PM, Maxim Patlasov wrote: On 12/01/2016 12:06 AM, Dmitry Monakhov wrote: Maxim Patlasov writes: Alexey, You're right. And while composing the patch I well understood that it's possible to rework fuse_sync_writes() using a counter instead of negative bias. But the problem with flush_mtime still exists anyway. Think about it: we firstly acquire local mtime from local inode, then fill and submit mtime-update-request. Since then, we don't know when exactly fuse daemon will apply that new mtime to its metadata structures. If another mtime-update is generated in-between (e.g. "touch -d file", or even simplier -- just a single direct write implicitly updating mtime), we wouldn't know which of those two mtime-update-requests are processed by fused first. That comes from a general FUSE protocol limitation: when kernel fuse queues request A, then request B, it cannot be sure if they will be processed by userspace as or . The big advantage of the patch I sent is that it's very simple, straightforward and presumably will remove 99% of contention between fsync and io_submit (assuming we spend most of time waiting for userspace ACK for FUSE_FSYNC request. There are actually three questions to answer: 1) Do we really must honor a crazy app who mixes a lot of fsyncs with a lot of io_submits? The goal of fsync is to ensure that some state is actually went to platters. An app who races io_submit-s with fsync-s actually doesn't care which state will come to platters. I'm not sure that it's reasonable to work very hard to achieve the best possible performance for such a marginal app. Obiously any filesystem behave like this. Task A(mail-server) may perform write/fsync, task B(mysql) do a lot of io_submit-s All that io may happens in parallel, fs guarantee only that metadata will be serialized. So all that concurent IO flowa to blockdevice which does no have i_mutex so all IO indeed happen concurrently. Looks as you're comparing an app doing POSIX open/read/write/fsync/close with fs doing submit_bio. This is a stretch. But OK, there is a similarity. But I don't think this rather vague similarity proves something. we are speaking about VM process, which essentially re-submits IO from the guest to host like above. For sure QEMU and VM_app have this IO pattern. Thus this pattern MUST be optimized as this is one of our main loads. Yes, I agree. That's exactly why I wrote in the same email (next paragraph): This really makes sense. If an app inside a VM loops over ordinary direct writes, while another app (in the same VM) does fsync, it's not fair to suspend the first app for long while just because fuse holds i_mutex for long somewhere deep in fuse_fsync. Max That is why I think that this case is not marginal and important. Den ___ Devel mailing list Devel@openvz.org https://lists.openvz.org/mailman/listinfo/devel
[Devel] [PATCH vz7] fuse: no mtime flush on fdatasync
fuse_fsync_common() may skip fuse_flush_mtime() if datasync=1 because
mtime is pure metadata and the content of file doesn't depend on it.
https://jira.sw.ru/browse/PSBM-55919
Signed-off-by: Maxim Patlasov
---
fs/fuse/file.c |4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 559dfd9..e5c4778 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -684,8 +684,8 @@ int fuse_fsync_common(struct file *file, loff_t start,
loff_t end,
if (err)
goto out;
- if (test_bit(FUSE_I_MTIME_UPDATED,
-&get_fuse_inode(inode)->state)) {
+ if (!datasync && test_bit(FUSE_I_MTIME_UPDATED,
+ &get_fuse_inode(inode)->state)) {
err = fuse_flush_mtime(file, false);
if (err)
goto out;
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