Re: [PATCH] mm, vmscan: do not loop on too_many_isolated for ever

[Tetsuo Handa]

Hugh Dickins wrote:
> On Thu, 20 Jul 2017, Tetsuo Handa wrote:
> > Hugh Dickins wrote:
> > > You probably won't welcome getting into alternatives at this late stage;
> > > but after hacking around it one way or another because of its pointless
> > > lockups, I lost patience with that too_many_isolated() loop a few months
> > > back (on realizing the enormous number of pages that may be isolated via
> > > migrate_pages(2)), and we've been running nicely since with something like:
> > > 
> > > 	bool got_mutex = false;
> > > 
> > > 	if (unlikely(too_many_isolated(pgdat, file, sc))) {
> > > 		if (mutex_lock_killable(&pgdat->too_many_isolated))
> > > 			return SWAP_CLUSTER_MAX;
> > > 		got_mutex = true;
> > > 	}
> > > 	...
> > > 	if (got_mutex)
> > > 		mutex_unlock(&pgdat->too_many_isolated);
> > > 
> > > Using a mutex to provide the intended throttling, without an infinite
> > > loop or an arbitrary delay; and without having to worry (as we often did)
> > > about whether those numbers in too_many_isolated() are really appropriate.
> > > No premature OOMs complained of yet.
> > 
> > Roughly speaking, there is a moment where shrink_inactive_list() acts
> > like below.
> > 
> > 	bool got_mutex = false;
> > 
> > 	if (!current_is_kswapd()) {
> > 		if (mutex_lock_killable(&pgdat->too_many_isolated))
> > 			return SWAP_CLUSTER_MAX;
> > 		got_mutex = true;
> > 	}
> > 
> > 	// kswapd is blocked here waiting for !current_is_kswapd().
> 
> That would be a shame, for kswapd to wait for !current_is_kswapd()!

Yes, but current code (not about your patch) does allow kswapd to wait
for memory allocations of !current_is_kswapd() thread to complete.

> 
> But seriously, I think I understand what you mean by that, you're
> thinking that kswapd would be waiting on some other task to clear
> the too_many_isolated() condition?

Yes.

> 
> No, it does not work that way: kswapd (never seeing too_many_isolated()
> because that always says false when current_is_kswapd()) never tries to
> take the pgdat->too_many_isolated mutex itself: it does not wait there
> at all, although other tasks may be waiting there at the time.

I know. I wrote behavior of your patch if my guess (your "..." part
corresponds to kswapd doing writepage) is correct.

> 
> Perhaps my naming the mutex "too_many_isolated", same as the function,
> is actually confusing, when I had intended it to be helpful.

Not confusing at all. It is helpful.
I just wanted to confirm what comes in your "..." part.

> 
> > 
> > 	if (got_mutex)
> > 		mutex_unlock(&pgdat->too_many_isolated);
> > 
> > > 
> > > But that was on a different kernel, and there I did have to make sure
> > > that PF_MEMALLOC always prevented us from nesting: I'm not certain of
> > > that in the current kernel (but do remember Johannes changing the memcg
> > > end to make it use PF_MEMALLOC too).  I offer the preview above, to see
> > > if you're interested in that alternative: if you are, then I'll go ahead
> > > and make it into an actual patch against v4.13-rc.
> > 
> > I don't know what your actual patch looks like, but the problem is that
> > pgdat->too_many_isolated waits for kswapd while kswapd waits for
> > pgdat->too_many_isolated; nobody can unlock pgdat->too_many_isolated if
> > once we hit it.
> 
> Not so (and we'd hardly be finding it a useful patch if that were so).

Current code allows kswapd to wait for memory allocation of !current_is_kswapd()
threads, and thus !current_is_kswapd() threads wait for current_is_kswapd() threads
while current_is_kswapd() threads wait for !current_is_kswapd() threads; nobody can
make too_many_isolated() false if once we hit it. Hence, this patch is proposed.

Thanks.