linux-kernel - RE: 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes]

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Date:   Fri, 6 Mar 2020 16:57:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        André Almeida <andrealmeid@...labora.com>
CC:     Florian Weimer <fweimer@...hat.com>,
        "Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel@...labora.com" <kernel@...labora.com>,
        "krisman@...labora.com" <krisman@...labora.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "ryao@...too.org" <ryao@...too.org>,
        "dvhart@...radead.org" <dvhart@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "z.figura12@...il.com" <z.figura12@...il.com>,
        "steven@...vesoftware.com" <steven@...vesoftware.com>,
        "steven@...uorix.net" <steven@...uorix.net>,
        "malteskarupke@....de" <malteskarupke@....de>,
        "carlos@...hat.com" <carlos@...hat.com>,
        "adhemerval.zanella@...aro.org" <adhemerval.zanella@...aro.org>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: RE: 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement
 mechanism to wait on any of several futexes]

From: Peter Zijlstra
> Sent: 05 March 2020 18:52
+> 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.

An extra copy_from_user is likely to be noticable.
It certainly makes recvmsg() slower than recv().
Especially if the hardended usercopy crap gets involved.

	David
 

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