On Thu, Jan 21, 2021 at 2:34 PM Paul Gortmaker <paul.gortma...@windriver.com> wrote: > > While this is done for all bitmaps, the original use case in mind was > for CPU masks and cpulist_parse(). Credit to Yury who suggested to > push it down from CPU subsys to bitmap - it simplified things a lot.
Can you convert your credit to Suggested-by or Reviewed-by? :) > It seems that a common configuration is to use the 1st couple cores > for housekeeping tasks, and or driving a busy peripheral that generates > a lot of interrupts, or something similar. > > This tends to leave the remaining ones to form a pool of similarly > configured cores to take on the real workload of interest to the user. > > So on machine A - with 32 cores, it could be 0-3 for "system" and then > 4-31 being used in boot args like nohz_full=, or rcu_nocbs= as part of > setting up the worker pool of CPUs. > > But then newer machine B is added, and it has 48 cores, and so while > the 0-3 part remains unchanged, the pool setup cpu list becomes 4-47. > > Deployment would be easier if we could just simply replace 31 and 47 > with "N" and let the system substitute in the actual number at boot; > a number that it knows better than we do. > > No need to have custom boot args per node, no need to do a trial boot > in order to snoop /proc/cpuinfo and/or /sys/devices/system/cpu - no > more fencepost errors of using 32 and 48 instead of 31 and 47. > > Cc: Yury Norov <yury.no...@gmail.com> > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: "Paul E. McKenney" <paul...@kernel.org> > Signed-off-by: Paul Gortmaker <paul.gortma...@windriver.com> > --- > .../admin-guide/kernel-parameters.rst | 4 ++++ > lib/bitmap.c | 18 +++++++++++++----- > 2 files changed, 17 insertions(+), 5 deletions(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.rst > b/Documentation/admin-guide/kernel-parameters.rst > index 5e080080b058..668f0b69fb4f 100644 > --- a/Documentation/admin-guide/kernel-parameters.rst > +++ b/Documentation/admin-guide/kernel-parameters.rst > @@ -68,6 +68,10 @@ For example one can add to the command line following > parameter: > > where the final item represents CPUs 100,101,125,126,150,151,... > > +The value "N" can be used as the end of a range, to represent the numerically > +last CPU on the system, i.e "foo_cpus=16-N" would be equivalent to "16-31" on > +a 32 core system. > + > The following convenience aliases are also accepted and used: > > foo_cpus=all > diff --git a/lib/bitmap.c b/lib/bitmap.c > index a1010646fbe5..d498ea9d526b 100644 > --- a/lib/bitmap.c > +++ b/lib/bitmap.c > @@ -571,7 +571,7 @@ static const char *bitmap_find_region_reverse(const char > *start, const char *end > return end; > } > > -static const char *bitmap_parse_region(const char *str, struct region *r) > +static const char *bitmap_parse_region(const char *str, struct region *r, > int nmaskbits) > { in bitmap_parselist() you can store nmaskbits in the struct region, and avoid passing nmaskbits as a parameter. > str = bitmap_getnum(str, &r->start); > if (IS_ERR(str)) > @@ -583,9 +583,15 @@ static const char *bitmap_parse_region(const char *str, > struct region *r) > if (*str != '-') > return ERR_PTR(-EINVAL); > > - str = bitmap_getnum(str + 1, &r->end); > - if (IS_ERR(str)) > - return str; > + str++; > + if (*str == 'N') { > + r->end = nmaskbits - 1; > + str++; > + } else { > + str = bitmap_getnum(str, &r->end); > + if (IS_ERR(str)) > + return str; > + } Indeed it's much simpler. But I don't like that you increase the nesting level. Can you keep bitmap_parse_region() a single-tab style function? What about group size? Are you going to support N there, like "0-N:5/N"? What about "N-N"? Is it legal? Maybe hide new logic in bitmap_getnum()? I would also like to see tests covering new functionality. As a user of "N", I want to be 100% sure that this "N" is a full equivalent of NR_CPUS, including error codes that the parser returns. Otherwise it will be hard to maintain the transition. > if (end_of_region(*str)) > goto no_pattern; > @@ -628,6 +634,8 @@ static const char *bitmap_parse_region(const char *str, > struct region *r) > * Syntax: range:used_size/group_size > * Example: 0-1023:2/256 ==> 0,1,256,257,512,513,768,769 > * Optionally the self-descriptive "all" or "none" can be used. > + * The value 'N' can be used as the end of a range to indicate the maximum > + * allowed value; i.e (nmaskbits - 1). > * > * Returns: 0 on success, -errno on invalid input strings. Error values: > * > @@ -656,7 +664,7 @@ int bitmap_parselist(const char *buf, unsigned long > *maskp, int nmaskbits) > if (buf == NULL) > return 0; > > - buf = bitmap_parse_region(buf, &r); > + buf = bitmap_parse_region(buf, &r, nmaskbits); > if (IS_ERR(buf)) > return PTR_ERR(buf); > > -- > 2.17.1 >