On Sun, Oct 23, 2016 at 06:57:26PM -0700, Davidlohr Bueso wrote:
> On Wed, 19 Oct 2016, Peter Zijlstra wrote:
>
> > Subject: sched: Better explain sleep/wakeup
> > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Date: Wed Oct 19 15:45:27 CEST 2016
> >
> > There were a few questions wrt how sleep-wakeup works. Try and explain
> > it more.
> >
> > Requested-by: Will Deacon <will.deacon@xxxxxxx>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> > include/linux/sched.h | 52 ++++++++++++++++++++++++++++++++++----------------
> > kernel/sched/core.c | 15 +++++++-------
> > 2 files changed, 44 insertions(+), 23 deletions(-)
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -262,20 +262,9 @@ extern char ___assert_task_state[1 - 2*!
> > #define set_task_state(tsk, state_value) \
> > do { \
> > (tsk)->task_state_change = _THIS_IP_; \
> > - smp_store_mb((tsk)->state, (state_value)); \
> > + smp_store_mb((tsk)->state, (state_value)); \
> > } while (0)
> >
> > -/*
> > - * set_current_state() includes a barrier so that the write of current->state
> > - * is correctly serialised wrt the caller's subsequent test of whether to
> > - * actually sleep:
> > - *
> > - * set_current_state(TASK_UNINTERRUPTIBLE);
> > - * if (do_i_need_to_sleep())
> > - * schedule();
> > - *
> > - * If the caller does not need such serialisation then use __set_current_state()
> > - */
> > #define __set_current_state(state_value) \
> > do { \
> > current->task_state_change = _THIS_IP_; \
> > @@ -284,11 +273,19 @@ extern char ___assert_task_state[1 - 2*!
> > #define set_current_state(state_value) \
> > do { \
> > current->task_state_change = _THIS_IP_; \
> > - smp_store_mb(current->state, (state_value)); \
> > + smp_store_mb(current->state, (state_value)); \
> > } while (0)
> >
> > #else
> >
> > +/*
> > + * @tsk had better be current, or you get to keep the pieces.
>
> That reminds me we were getting rid of the set_task_state() calls. Bcache was
> pending, being only user in the kernel that doesn't actually use current; but
> instead breaks newly (yet blocked/uninterruptible) created garbage collection
> kthread. I cannot figure out why this is done (ie purposely accounting the
> load avg. Furthermore gc kicks in in very specific scenarios obviously, such
> as as by the allocator task, so I don't see why bcache gc should want to be
> interruptible.
>
> Kent, Jens, can we get rid of this?
>
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index 76f7534d1dd1..6e3c358b5759 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -1798,7 +1798,6 @@ int bch_gc_thread_start(struct cache_set *c)
> if (IS_ERR(c->gc_thread))
> return PTR_ERR(c->gc_thread);
>
> - set_task_state(c->gc_thread, TASK_INTERRUPTIBLE);
> return 0;
> }
Actually, that code looks broken, or at least stupid. Let me do a proper fix...
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