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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jul 18, 2018 at 06:31:15PM +0200, Peter Zijlstra wrote:
> 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.

I'm not tied to that name, but I can't really think of a better
one. It was called PF_MEMDELAY in the past, but "delay" also has
busy-spinning connotations in the kernel. "wait" also implies that
it's a passive state.

> > > 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?

Yep, but not all of those, only the ones who were on the active list
in their previous incarnation, aka refaulting *hot* pages, aka there
is little chance this is healthy behavior.