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SubjectRe: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO
On Wed, Jul 18, 2018 at 06:36:44PM -0400, Johannes Weiner wrote:
> On Wed, Jul 18, 2018 at 02:03:18PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:
> > > + /* Time in which tasks wait for the CPU */
> > > + state = PSI_NONE;
> > > + if (tasks[NR_RUNNING] > 1)
> > > + state = PSI_SOME;
> > > + time_state(&groupc->res[PSI_CPU], state, now);
> > > +
> > > + /* Time in which tasks wait for memory */
> > > + state = PSI_NONE;
> > > + if (tasks[NR_MEMSTALL]) {
> > > + if (!tasks[NR_RUNNING] ||
> > > + (cpu_curr(cpu)->flags & PF_MEMSTALL))
> >
> > I'm confused, why do we care if the current tasks is MEMSTALL or not?
>
> We want to know whether we're losing CPU potential because of a lack
> of memory. That can happen when the task waits for refaults and the
> CPU goes idle, but it can also happen when the CPU is performing
> reclaim.
>
> If the task waits for refaults and something else is runnable, we're
> not losing CPU potential. But if the task performs reclaim and uses
> the CPU, nothing else can do productive work on that CPU.

Right, this is because MEMSTALL is not just blocking (as per that other
sub-thread).

This is really unfortunate, because it means the state is not a simple
function of the task counts.

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