Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

[Peter Zijlstra]

On Wed, Jul 18, 2018 at 09:56:33AM -0400, Johannes Weiner wrote:
> On Wed, Jul 18, 2018 at 02:46:27PM +0200, Peter Zijlstra wrote:

> > I'm confused by this whole MEMSTALL thing... I thought the idea was to
> > account the time we were _blocked_ because of memstall, but you seem to
> > count the time we're _running_ with PF_MEMSTALL.
> 
> Under heavy memory pressure, a lot of active CPU time is spent
> scanning and rotating through the LRU lists, which we do want to
> capture in the pressure metric. What we really want to know is the
> time in which CPU potential goes to waste due to a lack of
> resources. That's the CPU going idle due to a memstall, but it's also
> a CPU doing *work* which only occurs due to a lack of memory. We want
> to know about both to judge how productive system and workload are.

Then maybe memstall (esp. the 'stall' part of it) is a bit of a
misnomer.

> > And esp. the wait_on_page_bit_common caller seems performance sensitive,
> > and the above function is quite expensive.
> 
> Right, but we don't call it on every invocation, only when waiting for
> the IO to read back a page that was recently deactivated and evicted:
> 
> 	if (bit_nr == PG_locked &&
> 	    !PageUptodate(page) && PageWorkingset(page)) {
> 		if (!PageSwapBacked(page))
> 			delayacct_thrashing_start();
> 		psi_memstall_enter(&pflags);
> 		thrashing = true;
> 	}
> 
> That means the page cache workingset/file active list is thrashing, in
> which case the IO itself is our biggest concern, not necessarily a few
> additional cycles before going to sleep to wait on its completion.

Ah, right. PageWorkingset() is only true if we (recently) evicted that
page before, right?