From: Sebastian Andrzej Siewior <[email protected]>
To: [email protected]
Cc: [email protected], [email protected],
Ingo Molnar <[email protected]>,
Juri Lelli <[email protected]>,
Vincent Guittot <[email protected]>,
Dietmar Eggemann <[email protected]>,
Steven Rostedt <[email protected]>,
Ben Segall <[email protected]>, Mel Gorman <[email protected]>,
Jens Axboe <[email protected]>,
Thomas Gleixner <[email protected]>
Subject: Re: [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption
Date: Wed, 19 Aug 2020 15:33:20 +0200 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
On 2020-08-19 15:15:07 [+0200], [email protected] wrote:
> > - if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
> > + if (tsk->flags & PF_WQ_WORKER) {
> > preempt_disable();
> > - if (tsk->flags & PF_WQ_WORKER)
> > - wq_worker_sleeping(tsk);
> > - else
> > - io_wq_worker_sleeping(tsk);
> > + wq_worker_sleeping(tsk);
> > preempt_enable_no_resched();
> > }
> >
> > if (tsk_is_pi_blocked(tsk))
> > return;
> >
> > + if (tsk->flags & PF_IO_WORKER)
> > + io_wq_worker_sleeping(tsk);
> > +
>
> Urgh, so this adds a branch in what is normally considered a fairly hot
> path.
>
> I'm thinking that the raw_spinlock_t option would permit leaving that
> single:
>
> if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER))
>
> branch intact?
The compiler generates code to test for both flags at once. If none of
both possible flags are set then there is one branch (get out and bring
me to tst_is_pi…).
And yes, with raw_spinlock_t we could keep that one branch.
If you want to optimize further, we could move PF_IO_WORKER to an lower
bit. x86 can test for both via
(gcc-10)
| testl $536870944, 44(%rbp) #, _11->flags
| jne .L1635 #,
(clang-9)
| testl $536870944, 44(%rbx) # imm = 0x20000020
| je .LBB112_6
but ARM can't and does
| ldr r1, [r5, #16] @ tsk_3->flags, tsk_3->flags
| mov r2, #32 @ tmp157,
| movt r2, 8192 @ tmp157,
| tst r2, r1 @ tmp157, tsk_3->flags
| beq .L998 @,
same ARM64
| ldr w0, [x20, 60] //, _11->flags
| and w0, w0, 1073741792 // tmp117, _11->flags,
| and w0, w0, -536870849 // tmp117, tmp117,
| cbnz w0, .L453 // tmp117,
using 0x10 for PF_IO_WORKER instead will turn this into:
| ldr w0, [x20, 60] //, _11->flags
| tst w0, 48 // _11->flags,
| bne .L453 //,
ARM:
| ldr r2, [r5, #16] @ tsk_3->flags, tsk_3->flags
| tst r2, #48 @ tsk_3->flags,
| beq .L998 @,
Sebastian
next prev parent reply other threads:[~2020-08-19 13:33 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-19 12:37 [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption Sebastian Andrzej Siewior
2020-08-19 13:15 ` peterz
2020-08-19 13:18 ` Jens Axboe
2020-08-19 19:44 ` [PATCH] io_wq: Make io_wqe::lock a raw_spinlock_t Sebastian Andrzej Siewior
2020-09-01 8:41 ` [PATCH v2] " Sebastian Andrzej Siewior
2020-09-01 14:17 ` Jens Axboe
2020-08-19 13:33 ` Sebastian Andrzej Siewior [this message]
2020-08-19 14:21 ` [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption peterz
2020-08-19 19:55 ` [PATCH 1/2] sched: Bring the PF_IO_WORKER and PF_WQ_WORKER bits closer together Sebastian Andrzej Siewior
2020-08-19 20:00 ` [PATCH 2/2] sched: Cache task_struct::flags in sched_submit_work() Sebastian Andrzej Siewior
2020-08-19 20:11 ` Peter Zijlstra
2020-09-07 12:58 ` [PATCH 1/2] sched: Bring the PF_IO_WORKER and PF_WQ_WORKER bits closer together Will Deacon
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