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SubjectRe: [RFC PATCH 2/2] pipe: use pipe busy wait
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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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