Messages in this thread Patch in this message | | | Date | Tue, 6 Sep 2022 12:54:34 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match_state |
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On Sun, Sep 04, 2022 at 12:44:36PM +0200, Ingo Molnar wrote: > > * Peter Zijlstra <peterz@infradead.org> wrote: > > > Make wait_task_inactive()'s @match_state work like ttwu()'s @state. > > > > That is, instead of an equal comparison, use it as a mask. This allows > > matching multiple block conditions. > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > > --- > > kernel/sched/core.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -3295,7 +3295,7 @@ unsigned long wait_task_inactive(struct > > * is actually now running somewhere else! > > */ > > while (task_running(rq, p)) { > > - if (match_state && unlikely(READ_ONCE(p->__state) != match_state)) > > + if (match_state && !(READ_ONCE(p->__state) & match_state)) > > return 0; > > We lose the unlikely annotation there - but I guess it probably never > really mattered anyway?
So any wait_task_inactive() caller does want that case to be true, but the whole match_state precondition mostly wrecks things anyway. If anything it should've been:
if (likely(match_state && !(READ_ONCE(p->__state) & match_state))) return 0;
but I can't find it in me to care too much here.
> Suggestion #1: > > - Shouldn't we rename task_running() to something like task_on_cpu()? The > task_running() primitive is similar to TASK_RUNNING but is not based off > any TASK_FLAGS.
That looks like a simple enough patch, lemme go do that.
> Suggestion #2: > > - Shouldn't we eventually standardize on task->on_cpu on UP kernels too? > They don't really matter anymore, and doing so removes #ifdefs and makes > the code easier to read.
Probably, but that sounds like something that'll spiral out of control real quick, so I'll leave that on the TODO list somewhere.
> > cpu_relax(); > > } > > @@ -3310,7 +3310,7 @@ unsigned long wait_task_inactive(struct > > running = task_running(rq, p); > > queued = task_on_rq_queued(p); > > ncsw = 0; > > - if (!match_state || READ_ONCE(p->__state) == match_state) > > + if (!match_state || (READ_ONCE(p->__state) & match_state)) > > ncsw = p->nvcsw | LONG_MIN; /* sets MSB */ > > task_rq_unlock(rq, p, &rf); > > Suggestion #3: > > - Couldn't the following users with a 0 mask: > > drivers/powercap/idle_inject.c: wait_task_inactive(iit->tsk, 0); > fs/coredump.c: wait_task_inactive(ptr->task, 0); > > Use ~0 instead (exposed as TASK_ANY or so) and then we can drop the > !match_state special case? > > They'd do something like: > > drivers/powercap/idle_inject.c: wait_task_inactive(iit->tsk, TASK_ANY); > fs/coredump.c: wait_task_inactive(ptr->task, TASK_ANY); > > It's not an entirely 100% equivalent transformation though, but looks OK > at first sight: ->__state will be some nonzero mask for genuine tasks > waiting to schedule out, so any match will be functionally the same as a > 0 flag telling us not to check any of the bits, right? I might be missing > something though.
I too am thinking that should work. Added patch for that.
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