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From: Peter Zijlstra <peterz@infradead.org>
To: "André Almeida" <andrealmeid@collabora.com>
Cc: Florian Weimer <fweimer@redhat.com>,
	"Pierre-Loup A. Griffais" <pgriffais@valvesoftware.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, kernel@collabora.com,
	krisman@collabora.com, shuah@kernel.org,
	linux-kselftest@vger.kernel.org, rostedt@goodmis.org,
	ryao@gentoo.org, dvhart@infradead.org, mingo@redhat.com,
	z.figura12@gmail.com, steven@valvesoftware.com,
	steven@liquorix.net, malteskarupke@web.de, carlos@redhat.com,
	adhemerval.zanella@linaro.org, libc-alpha@sourceware.org,
	linux-api@vger.kernel.org
Subject: Re: 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes]
Date: Thu, 5 Mar 2020 19:51:36 +0100
Message-ID: <20200305185136.GB3348@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <52406c54-60b3-dcfe-65d8-4c425459e37b@collabora.com>

On Thu, Mar 05, 2020 at 01:14:17PM -0300, André Almeida wrote:

> >   sys_futex_wait(void *uaddr, u64 val, unsigned long flags, ktime_t *timo);
> >   struct futex_wait {
> > 	  void *uaddr;
> > 	  u64 val;
> > 	  u64 flags;
> >   };
> >   sys_futex_waitv(struct futex_wait *waiters, unsigned int nr_waiters,
> > 		  u64 flags, ktime_t *timo);
> >   sys_futex_wake(void *uaddr, unsigned int nr, u64 flags);
> >   sys_futex_cmp_requeue(void *uaddr1, void *uaddr2, unsigned int nr_wake,
> > 		  unsigned int nr_requeue, u64 cmpval, unsigned long flags);
> > 
> > And that makes 7 arguments for cmp_requeue, which can't be. Maybe we if
> > combine nr_wake and nr_requeue in one as 2 u16... ?
> > 
> > And then we need to go detector if the platform supports it or not..
> > 
> 
> Thanks everyone for the feedback around our mechanism. Are the
> performance benefits of implementing a syscall to wait on a single futex
> significant enough to maintain it instead of just using
> `sys_futex_waitv()` with `nr_waiters = 1`? If we join both cases in a
> single interface, we may even add a new member for NUMA hint in `struct
> futex_wait`.

My consideration was that avoiding the get_user/copy_from_user might
become measurable on !PTI systems with SMAP.

But someone would have to build it and measure it before we can be sure
of course.

  parent reply index

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-13 21:45 [PATCH v3 0/4] Implement FUTEX_WAIT_MULTIPLE operation André Almeida
2020-02-13 21:45 ` [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes André Almeida
2020-02-28 19:07   ` Peter Zijlstra
2020-02-28 19:49     ` Peter Zijlstra
2020-02-28 21:25       ` Thomas Gleixner
2020-02-29  0:29         ` Pierre-Loup A. Griffais
2020-02-29 10:27           ` Thomas Gleixner
2020-03-03  2:47             ` Pierre-Loup A. Griffais
2020-03-03 12:00               ` 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes] Peter Zijlstra
2020-03-03 13:00                 ` Florian Weimer
2020-03-03 13:21                   ` Peter Zijlstra
2020-03-03 13:47                     ` Florian Weimer
2020-03-03 15:01                       ` Peter Zijlstra
2020-03-05 16:14                         ` André Almeida
2020-03-05 16:25                           ` Florian Weimer
2020-03-05 18:51                           ` Peter Zijlstra [this message]
2020-03-06 16:57                             ` David Laight
2020-02-13 21:45 ` [PATCH v3 2/4] selftests: futex: Add FUTEX_WAIT_MULTIPLE timeout test André Almeida
2020-02-13 21:45 ` [PATCH v3 3/4] selftests: futex: Add FUTEX_WAIT_MULTIPLE wouldblock test André Almeida
2020-02-13 21:45 ` [PATCH v3 4/4] selftests: futex: Add FUTEX_WAIT_MULTIPLE wake up test André Almeida
2020-02-19 16:27 ` [PATCH v3 0/4] Implement FUTEX_WAIT_MULTIPLE operation shuah

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