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Date: Mon, 17 Sep 2018 18:07:41 -0700 From: Subhra Mazumdar <subhra.mazumdar@...cle.com> To: Peter Zijlstra <peterz@...radead.org> Cc: linux-kernel@...r.kernel.org, dhaval.giani@...cle.com, steven.sistare@...cle.com Subject: Re: [RFC PATCH 2/2] pipe: use pipe busy wait On 09/17/2018 03:43 PM, Peter Zijlstra wrote: > On Mon, Sep 17, 2018 at 02:05:40PM -0700, Subhra Mazumdar wrote: >> On 09/07/2018 05:25 AM, Peter Zijlstra wrote: >>> Why not just busy wait on current->state ? A little something like: >>> >>> diff --git a/fs/pipe.c b/fs/pipe.c >>> index bdc5d3c0977d..8d9f1c95ff99 100644 >>> --- a/fs/pipe.c >>> +++ b/fs/pipe.c >>> @@ -106,6 +106,7 @@ void pipe_double_lock(struct pipe_inode_info *pipe1, >>> void pipe_wait(struct pipe_inode_info *pipe) >>> { >>> DEFINE_WAIT(wait); >>> + u64 start; >>> /* >>> * Pipes are system-local resources, so sleeping on them >>> @@ -113,7 +114,15 @@ void pipe_wait(struct pipe_inode_info *pipe) >>> */ >>> prepare_to_wait(&pipe->wait, &wait, TASK_INTERRUPTIBLE); >>> pipe_unlock(pipe); >>> - schedule(); >>> + >>> + preempt_disable(); >>> + start = local_clock(); >>> + while (!need_resched() && current->state != TASK_RUNNING && >>> + (local_clock() - start) < pipe->poll_usec) >>> + cpu_relax(); >>> + schedule_preempt_disabled(); >>> + preempt_enable(); >>> + >>> finish_wait(&pipe->wait, &wait); >>> pipe_lock(pipe); >>> } >> This will make the current thread always spin and block as it itself does >> the state change to TASK_RUNNING in finish_wait. > Nah, the actual wakeup will also do that state change. The one in > finish_wait() is for the case where the wait condition became true > without wakeup, such that we don't 'leak' the INTERRUPTIBLE state. Ok, it works. I see similar improvements with hackbench as the original patch.
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