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SubjectRe: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure to make progress
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Hi, this is your Linux kernel regression tracker speaking.

On 02.12.21 16:06, Mel Gorman wrote:
> Mike Galbraith, Alexey Avramov and Darrick Wong all reported similar
> problems due to reclaim throttling for excessive lengths of time.
> In Alexey's case, a memory hog that should go OOM quickly stalls for
> several minutes before stalling. In Mike and Darrick's cases, a small
> memcg environment stalled excessively even though the system had enough
> memory overall.

Just wondering: this patch afaics is now in -mm and Linux next for
nearly two weeks. Is that intentional? I had expected it to be mainlined
with the batch of patches Andrew mailed to Linus last week, but it
wasn't among them.

Or am I missing something?

Ciao, Thorsten (wearing his 'Linux kernel regression tracker' hat)

P.S.: As a Linux kernel regression tracker I'm getting a lot of reports
on my table. I can only look briefly into most of them. Unfortunately
therefore I sometimes will get things wrong or miss something important.
I hope that's not the case here; if you think it is, don't hesitate to
tell me about it in a public reply, that's in everyone's interest.

BTW, I have no personal interest in this issue, which is tracked using
regzbot, my Linux kernel regression tracking bot
(https://linux-regtracking.leemhuis.info/regzbot/). I'm only posting
this mail to get things rolling again and hence don't need to be CC on
all further activities wrt to this regression.


> Commit 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being
> made") introduced the problem although commit a19594ca4a8b ("mm/vmscan:
> increase the timeout if page reclaim is not making progress") made it
> worse. Systems at or near an OOM state that cannot be recovered must
> reach OOM quickly and memcg should kill tasks if a memcg is near OOM.
>
> To address this, only stall for the first zone in the zonelist, reduce
> the timeout to 1 tick for VMSCAN_THROTTLE_NOPROGRESS and only stall if
> the scan control nr_reclaimed is 0, kswapd is still active and there were
> excessive pages pending for writeback. If kswapd has stopped reclaiming due
> to excessive failures, do not stall at all so that OOM triggers relatively
> quickly. Similarly, if an LRU is simply congested, only lightly throttle
> similar to NOPROGRESS.
>
> Alexey's original case was the most straight forward
>
> for i in {1..3}; do tail /dev/zero; done
>
> On vanilla 5.16-rc1, this test stalled heavily, after the patch the test
> completes in a few seconds similar to 5.15.
>
> Alexey's second test case added watching a youtube video while tail runs
> 10 times. On 5.15, playback only jitters slightly, 5.16-rc1 stalls a lot
> with lots of frames missing and numerous audio glitches. With this patch
> applies, the video plays similarly to 5.15.
>
> Link: https://lore.kernel.org/r/99e779783d6c7fce96448a3402061b9dc1b3b602.camel@gmx.de
> Link: https://lore.kernel.org/r/20211124011954.7cab9bb4@mail.inbox.lv
> Link: https://lore.kernel.org/r/20211022144651.19914-1-mgorman@techsingularity.net
>
> [lkp@intel.com: Fix W=1 build warning]
> Reported-and-tested-by: Alexey Avramov <hakavlad@inbox.lv>
> Reported-and-tested-by: Mike Galbraith <efault@gmx.de>
> Reported-and-tested-by: Darrick J. Wong <djwong@kernel.org>
> Reported-by: kernel test robot <lkp@intel.com>
> Fixes: 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being made")
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> include/linux/mmzone.h | 1 +
> include/trace/events/vmscan.h | 4 ++-
> mm/vmscan.c | 64 ++++++++++++++++++++++++++++++-----
> 3 files changed, 59 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 58e744b78c2c..936dc0b6c226 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -277,6 +277,7 @@ enum vmscan_throttle_state {
> VMSCAN_THROTTLE_WRITEBACK,
> VMSCAN_THROTTLE_ISOLATED,
> VMSCAN_THROTTLE_NOPROGRESS,
> + VMSCAN_THROTTLE_CONGESTED,
> NR_VMSCAN_THROTTLE,
> };
>
> diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
> index f25a6149d3ba..ca2e9009a651 100644
> --- a/include/trace/events/vmscan.h
> +++ b/include/trace/events/vmscan.h
> @@ -30,12 +30,14 @@
> #define _VMSCAN_THROTTLE_WRITEBACK (1 << VMSCAN_THROTTLE_WRITEBACK)
> #define _VMSCAN_THROTTLE_ISOLATED (1 << VMSCAN_THROTTLE_ISOLATED)
> #define _VMSCAN_THROTTLE_NOPROGRESS (1 << VMSCAN_THROTTLE_NOPROGRESS)
> +#define _VMSCAN_THROTTLE_CONGESTED (1 << VMSCAN_THROTTLE_CONGESTED)
>
> #define show_throttle_flags(flags) \
> (flags) ? __print_flags(flags, "|", \
> {_VMSCAN_THROTTLE_WRITEBACK, "VMSCAN_THROTTLE_WRITEBACK"}, \
> {_VMSCAN_THROTTLE_ISOLATED, "VMSCAN_THROTTLE_ISOLATED"}, \
> - {_VMSCAN_THROTTLE_NOPROGRESS, "VMSCAN_THROTTLE_NOPROGRESS"} \
> + {_VMSCAN_THROTTLE_NOPROGRESS, "VMSCAN_THROTTLE_NOPROGRESS"}, \
> + {_VMSCAN_THROTTLE_CONGESTED, "VMSCAN_THROTTLE_CONGESTED"} \
> ) : "VMSCAN_THROTTLE_NONE"
>
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fb9584641ac7..4c4d5f6cd8a3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1021,6 +1021,39 @@ static void handle_write_error(struct address_space *mapping,
> unlock_page(page);
> }
>
> +static bool skip_throttle_noprogress(pg_data_t *pgdat)
> +{
> + int reclaimable = 0, write_pending = 0;
> + int i;
> +
> + /*
> + * If kswapd is disabled, reschedule if necessary but do not
> + * throttle as the system is likely near OOM.
> + */
> + if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
> + return true;
> +
> + /*
> + * If there are a lot of dirty/writeback pages then do not
> + * throttle as throttling will occur when the pages cycle
> + * towards the end of the LRU if still under writeback.
> + */
> + for (i = 0; i < MAX_NR_ZONES; i++) {
> + struct zone *zone = pgdat->node_zones + i;
> +
> + if (!populated_zone(zone))
> + continue;
> +
> + reclaimable += zone_reclaimable_pages(zone);
> + write_pending += zone_page_state_snapshot(zone,
> + NR_ZONE_WRITE_PENDING);
> + }
> + if (2 * write_pending <= reclaimable)
> + return true;
> +
> + return false;
> +}
> +
> void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
> {
> wait_queue_head_t *wqh = &pgdat->reclaim_wait[reason];
> @@ -1056,8 +1089,16 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
> }
>
> break;
> + case VMSCAN_THROTTLE_CONGESTED:
> + fallthrough;
> case VMSCAN_THROTTLE_NOPROGRESS:
> - timeout = HZ/2;
> + if (skip_throttle_noprogress(pgdat)) {
> + cond_resched();
> + return;
> + }
> +
> + timeout = 1;
> +
> break;
> case VMSCAN_THROTTLE_ISOLATED:
> timeout = HZ/50;
> @@ -3321,7 +3362,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> if (!current_is_kswapd() && current_may_throttle() &&
> !sc->hibernation_mode &&
> test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))
> - reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
> + reclaim_throttle(pgdat, VMSCAN_THROTTLE_CONGESTED);
>
> if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
> sc))
> @@ -3386,16 +3427,16 @@ static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
> }
>
> /*
> - * Do not throttle kswapd on NOPROGRESS as it will throttle on
> - * VMSCAN_THROTTLE_WRITEBACK if there are too many pages under
> - * writeback and marked for immediate reclaim at the tail of
> - * the LRU.
> + * Do not throttle kswapd or cgroup reclaim on NOPROGRESS as it will
> + * throttle on VMSCAN_THROTTLE_WRITEBACK if there are too many pages
> + * under writeback and marked for immediate reclaim at the tail of the
> + * LRU.
> */
> - if (current_is_kswapd())
> + if (current_is_kswapd() || cgroup_reclaim(sc))
> return;
>
> /* Throttle if making no progress at high prioities. */
> - if (sc->priority < DEF_PRIORITY - 2)
> + if (sc->priority == 1 && !sc->nr_reclaimed)
> reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
> }
>
> @@ -3415,6 +3456,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> unsigned long nr_soft_scanned;
> gfp_t orig_mask;
> pg_data_t *last_pgdat = NULL;
> + pg_data_t *first_pgdat = NULL;
>
> /*
> * If the number of buffer_heads in the machine exceeds the maximum
> @@ -3478,14 +3520,18 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> /* need some check for avoid more shrink_zone() */
> }
>
> + if (!first_pgdat)
> + first_pgdat = zone->zone_pgdat;
> +
> /* See comment about same check for global reclaim above */
> if (zone->zone_pgdat == last_pgdat)
> continue;
> last_pgdat = zone->zone_pgdat;
> shrink_node(zone->zone_pgdat, sc);
> - consider_reclaim_throttle(zone->zone_pgdat, sc);
> }
>
> + consider_reclaim_throttle(first_pgdat, sc);
> +
> /*
> * Restore to original mask to avoid the impact on the caller if we
> * promoted it to __GFP_HIGHMEM.

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