linux-kernel - Re: [PATCH glibc 6/9] glibc: sched_getcpu(): use rseq cpu

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Message-ID: <87a72xz3ya.fsf@mid.deneb.enyo.de>
Date:   Mon, 27 Apr 2020 11:13:33 +0200
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Mathieu Desnoyers via Libc-alpha <libc-alpha@...rceware.org>
Cc:     Carlos O'Donell <carlos@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-api@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ben Maurer <bmaurer@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Paul Turner <pjt@...gle.com>,
        Joseph Myers <joseph@...esourcery.com>
Subject: Re: [PATCH glibc 6/9] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v7)

* Mathieu Desnoyers via Libc-alpha:

> diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
> index c019cfb3cf..2269c4f2bd 100644
> --- a/sysdeps/unix/sysv/linux/sched_getcpu.c
> +++ b/sysdeps/unix/sysv/linux/sched_getcpu.c
> @@ -18,10 +18,15 @@
>  #include <errno.h>
>  #include <sched.h>
>  #include <sysdep.h>
> +#include <atomic.h>
>  #include <sysdep-vdso.h>
>  
> -int
> -sched_getcpu (void)
> +#ifdef HAVE_GETCPU_VSYSCALL
> +# define HAVE_VSYSCALL
> +#endif
> +
> +static int
> +vsyscall_sched_getcpu (void)
>  {
>    unsigned int cpu;
>    int r = -1;
> @@ -32,3 +37,21 @@ sched_getcpu (void)
>  #endif
>    return r == -1 ? r : cpu;
>  }
> +
> +#include <sys/rseq.h>
> +
> +#ifdef RSEQ_SIG
> +int
> +sched_getcpu (void)
> +{
> +  int cpu_id = atomic_load_relaxed (&__rseq_abi.cpu_id);
> +
> +  return cpu_id >= 0 ? cpu_id : vsyscall_sched_getcpu ();
> +}
> +#else
> +int
> +sched_getcpu (void)
> +{
> +  return vsyscall_sched_getcpu ();
> +}
> +#endif

This patch needs to be rebased on current master, but the direction
looks right to me.