On Tue, Apr 28, 2015 at 04:13:15PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 27, 2015 at 12:51:01PM +0800, Yuanhan Liu wrote:
> > It's just a variant of wait_event_cmd, with exclusive flag being set.
> >
> > For cases like RAID5, which puts many processes to sleep until 1/4
> > resources are free, a wake_up wakes up all processes to run, but
> > there is one process being able to get the resource as it's protected
> > by a spin lock. That ends up introducing heavy lock contentions, and
> > hurts performance badly.
> >
> > Here introduce wait_event_cmd_exclusive to relieve the lock contention
> > naturally by letting wake_up() just wake up one process.
> >
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Yuanhan Liu <yuanhan.liu@xxxxxxxxxxxxxxx>
> > ---
> > include/linux/wait.h | 14 +++++++++++---
> > 1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/wait.h b/include/linux/wait.h
> > index 2db8334..6c3b4de 100644
> > --- a/include/linux/wait.h
> > +++ b/include/linux/wait.h
> > @@ -358,10 +358,18 @@ do { \
> > __ret; \
> > })
> >
> > -#define __wait_event_cmd(wq, condition, cmd1, cmd2) \
> > - (void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, 0, \
> > +#define __wait_event_cmd(wq, condition, cmd1, cmd2, exclusive) \
> > + (void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, exclusive, 0, \
> > cmd1; schedule(); cmd2)
> >
> > +
> > +#define wait_event_cmd_exclusive(wq, condition, cmd1, cmd2) \
> > +do { \
> > + if (condition) \
> > + break; \
> > + __wait_event_cmd(wq, condition, cmd1, cmd2, 1); \
> > +} while (0)
> > +
> > /**
> > * wait_event_cmd - sleep until a condition gets true
> > * @wq: the waitqueue to wait on
> > @@ -380,7 +388,7 @@ do { \
> > do { \
> > if (condition) \
> > break; \
> > - __wait_event_cmd(wq, condition, cmd1, cmd2); \
> > + __wait_event_cmd(wq, condition, cmd1, cmd2, 0); \
> > } while (0)
> >
>
> No, that's wrong, its assumed that wait*() and __wait*() have the same
> arguments.
Thanks. Will send an updated patch soon.
--yliu
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