From: Florian Weimer <fweimer@redhat.com>
To: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
mingo@redhat.com, linux-kernel@vger.kernel.org,
linux-api@vger.kernel.org
Subject: Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs
Date: Mon, 08 Apr 2019 09:49:35 +0200 [thread overview]
Message-ID: <878swlszqo.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20190406194825.GA5106@avx2> (Alexey Dobriyan's message of "Sat, 6 Apr 2019 22:48:25 +0300")
* Alexey Dobriyan:
>> >> 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).
>>
>> Given that old kernels fail with EINVAL, that evidence is fairly
>> restricted.
>>
>> I'm not sure if it's a good idea to overload this interface. I expect
>> that users will want to call sched_getaffinity (the system call wrapper)
>> with cpusetsize == 0 to query the value, so there will be pressure on
>> glibc to remove the memset. At that point we have an API that obscurely
>> fails with old glibc versions, but suceeds with newer ones, which isn't
>> great.
>
> I can do "if (len == 536870912)" so that bit count overflows on old
> kernels into EINVAL and is unlikely to be used ever.
I don't see how this solves this particular issue. It will still result
in a mysterious crash if programs use an updated system call wrapper.
Thanks,
Florian
prev parent reply other threads:[~2019-04-08 7:49 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-03 20:08 [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs Alexey Dobriyan
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 [this message]
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:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=878swlszqo.fsf@oldenburg2.str.redhat.com \
--to=fweimer@redhat.com \
--cc=adobriyan@gmail.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).