Re: [PATCH 5/6] mm/vmscan: Don't change pgdat state on base of a single LRU list state.

Andrey Ryabinin
2018-03-21T10:40:32Z

On 03/20/2018 06:25 PM, Michal Hocko wrote:
> On Thu 15-03-18 19:45:52, Andrey Ryabinin wrote:
>> We have separate LRU list for each memory cgroup. Memory reclaim iterates
>> over cgroups and calls shrink_inactive_list() every inactive LRU list.
>> Based on the state of a single LRU shrink_inactive_list() may flag
>> the whole node as dirty,congested or under writeback. This is obviously
>> wrong and hurtful. It's especially hurtful when we have possibly
>> small congested cgroup in system. Than *all* direct reclaims waste time
>> by sleeping in wait_iff_congested().
>
> I assume you have seen this in real workloads. Could you be more
> specific about how you noticed the problem?
>

Does it matter? One of our userspace processes have some sort of watchdog.
When it doesn't receive some event in time it complains that process stuck.
In this case in-kernel allocation stuck in wait_iff_congested.

>> Sum reclaim stats across all visited LRUs on node and flag node as dirty,
>> congested or under writeback based on that sum. This only fixes the
>> problem for global reclaim case. Per-cgroup reclaim will be addressed
>> separately by the next patch.
>>
>> This change will also affect systems with no memory cgroups. Reclaimer
>> now makes decision based on reclaim stats of the both anon and file LRU
>> lists. E.g. if the file list is in congested state and get_scan_count()
>> decided to reclaim some anon pages, reclaimer will start shrinking
>> anon without delay in wait_iff_congested() like it was before. It seems
>> to be a reasonable thing to do. Why waste time sleeping, before reclaiming
>> anon given that we going to try to reclaim it anyway?
>
> Well, if we have few anon pages in the mix then we stop throttling the
> reclaim, I am afraid. I am worried this might get us kswapd hogging CPU
> problems back.
>

Yeah, it's not ideal choice. If only few anon pages taken than *not* throttling is bad,
and if few file pages taken and many anon than *not* throttling is probably good.

Anyway, such requires more thought,research,justification, etc.
I'll change the patch to take into account file only pages, as it was before the patch.

> [...]

>> @@ -2579,6 +2542,58 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>> if (sc->nr_reclaimed - nr_reclaimed)
>> reclaimable = true;
>>
>> + /*
>> + * If reclaim is isolating dirty pages under writeback, it implies
>> + * that the long-lived page allocation rate is exceeding the page
>> + * laundering rate. Either the global limits are not being effective
>> + * at throttling processes due to the page distribution throughout
>> + * zones or there is heavy usage of a slow backing device. The
>> + * only option is to throttle from reclaim context which is not ideal
>> + * as there is no guarantee the dirtying process is throttled in the
>> + * same way balance_dirty_pages() manages.
>> + *
>> + * Once a node is flagged PGDAT_WRITEBACK, kswapd will count the number
>> + * of pages under pages flagged for immediate reclaim and stall if any
>> + * are encountered in the nr_immediate check below.
>> + */
>> + if (stat.nr_writeback && stat.nr_writeback == stat.nr_taken)
>> + set_bit(PGDAT_WRITEBACK, &pgdat->flags);
>> +
>> + /*
>> + * Legacy memcg will stall in page writeback so avoid forcibly
>> + * stalling here.
>> + */
>> + if (sane_reclaim(sc)) {
>> + /*
>> + * Tag a node as congested if all the dirty pages scanned were
>> + * backed by a congested BDI and wait_iff_congested will stall.
>> + */
>> + if (stat.nr_dirty && stat.nr_dirty == stat.nr_congested)
>> + set_bit(PGDAT_CONGESTED, &pgdat->flags);
>> +
>> + /* Allow kswapd to start writing pages during reclaim. */
>> + if (stat.nr_unqueued_dirty == stat.nr_taken)
>> + set_bit(PGDAT_DIRTY, &pgdat->flags);
>> +
>> + /*
>> + * If kswapd scans pages marked marked for immediate
>> + * reclaim and under writeback (nr_immediate), it implies
>> + * that pages are cycling through the LRU faster than
>> + * they are written so also forcibly stall.
>> + */
>> + if (stat.nr_immediate)
>> + congestion_wait(BLK_RW_ASYNC, HZ/10);
>> + }
>> +
>> + /*
>> + * Stall direct reclaim for IO completions if underlying BDIs and node
>> + * is congested. Allow kswapd to continue until it starts encountering
>> + * unqueued dirty pages or cycling through the LRU too quickly.
>> + */
>> + if (!sc->hibernation_mode && !current_is_kswapd() &&
>> + current_may_throttle())
>> + wait_iff_congested(pgdat, BLK_RW_ASYNC, HZ/10);
>> +
>> } while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
>> sc->nr_scanned - nr_scanned, sc));
>
> Why didn't you put the whole thing after the loop?
>

Why this should be put after the loop? Here we already scanned all LRUs on node and
can decide in what state the node is. If should_countinue_reclaim() decides to continue,
the reclaim will be continued in accordance to the state of the node.