archive mirror
 help / color / mirror / Atom feed
From: Alexey Dobriyan <>
Subject: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs
Date: Wed, 3 Apr 2019 23:08:09 +0300	[thread overview]
Message-ID: <20190403200809.GA13876@avx2> (raw)

Currently there is no easy way to get the number of CPUs on the system.
Applications are divided into 2 groups:
One group allocates buffer and call sched_getaffinity(2) once. It works
but either underallocate or overallocates and in the future such application
will become buggy as Linux will start working on even more SMP-ier systems.

Glibc in particular shipped with 1024 CPUs support maximum at some point
which is quite surprising as glibc maitainers should know better.

Another group dynamically grow buffer until cpumask fits. This is
inefficient as multiple system calls are done.

Nobody seems to parse "/sys/devices/system/cpu/possible".
Even if someone does, parsing sysfs is much slower than necessary.

Patch overloads sched_getaffinity(len=0) to simply return "nr_cpu_ids".
This will make gettting CPU mask require at most 2 system calls
and will eliminate unnecessary code.

len=0 is chosen so that
* passing zeroes is the simplest thing

	syscall(__NR_sched_getaffinity, 0, 0, NULL)

  will simply do the right thing,

* old kernels returned -EINVAL unconditionally.

Note: glibc segfaults upon exiting from system call because it tries to
clear the rest of the buffer if return value is positive, so
applications will have to use syscall(3).
Good news is that it proves noone uses sched_getaffinity(pid, 0, NULL).

Signed-off-by: Alexey Dobriyan <>

 kernel/sched/core.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4942,6 +4942,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
 	int ret;
 	cpumask_var_t mask;
+	if (len == 0)
+		return nr_cpu_ids;
 	if ((len * BITS_PER_BYTE) < nr_cpu_ids)
 		return -EINVAL;
 	if (len & (sizeof(unsigned long)-1))

             reply	other threads:[~2019-04-03 20:08 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-03 20:08 Alexey Dobriyan [this message]
2019-04-04  8:42 ` Peter Zijlstra
2019-04-04 18:02   ` Alexey Dobriyan
2019-04-05  9:26     ` Peter Zijlstra
2019-04-05 10:16   ` Florian Weimer
2019-04-05 11:04     ` Peter Zijlstra
2019-04-05 11:08       ` Florian Weimer
2019-04-05 11:49         ` Peter Zijlstra
2019-04-06 19:48     ` Alexey Dobriyan
2019-04-08  7:49       ` Florian Weimer

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190403200809.GA13876@avx2 \ \ \ \ \
    --subject='Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox