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 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?

Best Regards,
Huang, Ying