Re: extending wait4(2) or waitid(2) linux syscall

[hpa at zytor dot com]

On November 15, 2018 7:30:11 AM PST, "Dmitry V. Levin" <ldv@altlinux.org> wrote:
>On Thu, Nov 15, 2018 at 06:39:03AM -0800, Arnd Bergmann wrote:
>> On Thu, Nov 15, 2018 at 6:05 AM Dmitry V. Levin wrote:
>> > On Thu, Apr 20, 2017 at 03:20:51PM +0200, Albert ARIBAUD wrote:
>[...]
>> > > https://sourceware.org/glibc/wiki/Y2038ProofnessDesign?rev=146
>> > Is there any rationale for marking wait4 as an obsolete API?
>> 
>> In the *kernel* syscall API, wait4(2) is obsoleted by waitid(2),
>which is
>> a strict superset of its functionality.
>> 
>> In the libc API, this is different, as wait4() does not have a
>replacement
>> that is exposed to user space directly. I expect glibc to implement
>> wait4() on top of the kernel's waitid().
>> 
>> There has not been a final decision on which variant of waitid() that
>would
>> be. The easiest option would be to not change it at all: new
>architectures
>> (rv32, csky, nanomips/p32, ...) would keep exposing the traditional
>> waitid() in Linux, with its 32-bit time_t based rusage structure, but
>drop the
>> wait4(). glibc then has to convert between the kernel's rusage and
>the
>> user space rusage indefinitely.
>> 
>> Alternatively, we can create a new version like waitid2() that uses
>> 64-bit time_t in some form, either the exact same rusage that we
>> use on 64-bit architectures and x32, or using a new set of arguments
>> to include further improvements.
>
>In strace, we have two use cases that require an extended version
>of wait4(2) or waitid(2) syscall.  From your response I understand that
>you'd recommend extending waitid(2) rather than wait4(2), is it
>correct?
>
>These two use cases were mentioned in my talk yesterday at LPC 2018,
>here is a brief summary.
>
>1. strace needs a race-free invocation of wait4(2) or waitid(2)
>with a different signal mask, this cannot be achieved without
>an extended version of syscall, similar to pselect6(2) extension
>over select(2) and ppoll(2) extension over poll(2).
>
>Signal mask specification in linux requires two parameters:
>"const sigset_t *sigmask" and "size_t sigsetsize".
>Creating pwait6(2) as an extension of wait4(2) with two arguments
>is straightforward.
>Creating pwaitid(2) as an extension of waitid(2) that already has 5
>arguments would require an indirection similar to pselect6(2).
>
>2. The time precision provided by struct rusage returned by wait4(2)
>and
>waitid(2) is too low for syscall time counting (strace -c) nowadays,
>this
>can be observing by running in a row a simple command like "strace -c
>pwd".
>
>The fix is to return a more appropriate structure than struct rusage
>by the new pwait6(2)/pwaitid(2) syscall mentioned above, where
>struct timeval is replaced with struct timespec or even struct
>timespec64.

Arnd: w.r.t. our previous discussion, this would seem to justify going to timespec(64) for these kind of cases.
-- 
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