Yu Zhao <yuz...@google.com> writes:
> On Tue, Mar 16, 2021 at 10:08:51AM +0800, Huang, Ying wrote:
>> Yu Zhao <yuz...@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?
Best Regards,
Huang, Ying
