Re: [PATCH v1 10/14] mm: multigenerational lru: core

["Huang, Ying" <ying.huang@intel.com>]

Yu Zhao <yuzhao@google.com> writes:

> On Tue, Mar 16, 2021 at 02:52:52PM +0800, Huang, Ying wrote:
>> Yu Zhao <yuzhao@google.com> writes:
>> 
>> > On Tue, Mar 16, 2021 at 10:08:51AM +0800, Huang, Ying wrote:
>> >> Yu Zhao <yuzhao@google.com> writes:
>> >> [snip]
>> >> 
>> >> > +/* Main function used by foreground, background and user-triggered aging. */
>> >> > +static bool walk_mm_list(struct lruvec *lruvec, unsigned long next_seq,
>> >> > +			 struct scan_control *sc, int swappiness)
>> >> > +{
>> >> > +	bool last;
>> >> > +	struct mm_struct *mm = NULL;
>> >> > +	int nid = lruvec_pgdat(lruvec)->node_id;
>> >> > +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>> >> > +	struct lru_gen_mm_list *mm_list = get_mm_list(memcg);
>> >> > +
>> >> > +	VM_BUG_ON(next_seq > READ_ONCE(lruvec->evictable.max_seq));
>> >> > +
>> >> > +	/*
>> >> > +	 * For each walk of the mm list of a memcg, we decrement the priority
>> >> > +	 * of its lruvec. For each walk of memcgs in kswapd, we increment the
>> >> > +	 * priorities of all lruvecs.
>> >> > +	 *
>> >> > +	 * So if this lruvec has a higher priority (smaller value), it means
>> >> > +	 * other concurrent reclaimers (global or memcg reclaim) have walked
>> >> > +	 * its mm list. Skip it for this priority to balance the pressure on
>> >> > +	 * all memcgs.
>> >> > +	 */
>> >> > +#ifdef CONFIG_MEMCG
>> >> > +	if (!mem_cgroup_disabled() && !cgroup_reclaim(sc) &&
>> >> > +	    sc->priority > atomic_read(&lruvec->evictable.priority))
>> >> > +		return false;
>> >> > +#endif
>> >> > +
>> >> > +	do {
>> >> > +		last = get_next_mm(lruvec, next_seq, swappiness, &mm);
>> >> > +		if (mm)
>> >> > +			walk_mm(lruvec, mm, swappiness);
>> >> > +
>> >> > +		cond_resched();
>> >> > +	} while (mm);
>> >> 
>> >> It appears that we need to scan the whole address space of multiple
>> >> processes in this loop?
>> >> 
>> >> If so, I have some concerns about the duration of the function.  Do you
>> >> have some number of the distribution of the duration of the function?
>> >> And may be the number of mm_struct and the number of pages scanned.
>> >> 
>> >> In comparison, in the traditional LRU algorithm, for each round, only a
>> >> small subset of the whole physical memory is scanned.
>> >
>> > Reasonable concerns, and insightful too. We are sensitive to direct
>> > reclaim latency, and we tuned another path carefully so that direct
>> > reclaims virtually don't hit this path :)
>> >
>> > Some numbers from the cover letter first:
>> >   In addition, direct reclaim latency is reduced by 22% at 99th
>> >   percentile and the number of refaults is reduced 7%. These metrics are
>> >   important to phones and laptops as they are correlated to user
>> >   experience.
>> >
>> > And "another path" is the background aging in kswapd:
>> >   age_active_anon()
>> >     age_lru_gens()
>> >       try_walk_mm_list()
>> >         /* try to spread pages out across spread+1 generations */
>> >         if (old_and_young[0] >= old_and_young[1] * spread &&
>> >             min_nr_gens(max_seq, min_seq, swappiness) > max(spread, MIN_NR_GENS))
>> >                 return;
>> >
>> >         walk_mm_list(lruvec, max_seq, sc, swappiness);
>> >
>> > By default, spread = 2, which makes kswapd slight more aggressive
>> > than direct reclaim for our use cases. This can be entirely disabled
>> > by setting spread to 0, for worloads that don't care about direct
>> > reclaim latency, or larger values, they are more sensitive than
>> > ours.
>> 
>> OK, I see.  That can avoid the long latency in direct reclaim path.
>> 
>> > It's worth noting that walk_mm_list() is multithreaded -- reclaiming
>> > threads can work on different mm_structs on the same list
>> > concurrently. We do occasionally see this function in direct reclaims,
>> > on over-overcommitted systems, i.e., kswapd CPU usage is 100%. Under
>> > the same condition, we saw the current page reclaim live locked and
>> > triggered hardware watchdog timeouts (our hardware watchdog is set to
>> > 2 hours) many times.
>> 
>> Just to confirm, in the current page reclaim, kswapd will keep running
>> until watchdog?  This is avoided in your algorithm mainly via
>> multi-threading?  Or via direct vs. reversing page table scanning?
>
> Well, don't tell me you've seen the problem :) Let me explain one
> subtle difference in how the aging works between the current page
> reclaim and this series, and point you to the code.
>
> In the current page reclaim, we can't scan a page via the rmap without
> isolating the page first. So the aging basically isolates a batch of
> pages from a lru list, walks the rmap for each of the pages, and puts
> active ones back to the list.
>
> In this series, aging walks page tables to update the generation
> numbers of active pages without isolating them. The isolation is the
> subtle difference: it's not a problem when there are few threads, but
> it causes live locks when hundreds of threads running the aging and
> hit the following in shrink_inactive_list():
>
> 	while (unlikely(too_many_isolated(pgdat, file, sc))) {
> 		if (stalled)
> 			return 0;
>
> 		/* wait a bit for the reclaimer. */
> 		msleep(100);
> 		stalled = true;
>
> 		/* We are about to die and free our memory. Return now. */
> 		if (fatal_signal_pending(current))
> 			return SWAP_CLUSTER_MAX;
> 	}
>
> Thanks to Michal who has improved it considerably by commit
> db73ee0d4637 ("mm, vmscan: do not loop on too_many_isolated for
> ever"). But we still occasionally see live locks on over-overcommitted
> machines. Reclaiming threads step on each other while interleaving
> between the msleep() and the aging, on 100+ CPUs.

Got it!  Thanks a lot for detailed explanation!

Best Regards,
Huang, Ying