mm: vmscan: disable memcg direct reclaim stalling if cgroup writeback support is in use
Because writeback wasn't cgroup aware before, the usual dirty
throttling mechanism in balance_dirty_pages() didn't work for
processes under memcg limit. The writeback path didn't know how much
memory is available or how fast the dirty pages are being written out
for a given memcg and balance_dirty_pages() didn't have any measure of
IO back pressure for the memcg.
To work around the issue, memcg implemented an ad-hoc dirty throttling
mechanism in the direct reclaim path by stalling on pages under
writeback which are encountered during direct reclaim scan. This is
rather ugly and crude - none of the configurability, fairness, or
bandwidth-proportional distribution of the normal path.
The previous patches implemented proper memcg aware dirty throttling
when cgroup writeback is in use making the ad-hoc mechanism
unnecessary. This patch disables direct reclaim stalling for such
case.
Note: I disabled the parts which seemed obvious and it behaves fine
while testing but my understanding of this code path is
rudimentary and it's quite possible that I got something wrong.
Please let me know if I got some wrong or more global_reclaim()
sites should be updated.
v2: The original patch removed the direct stalling mechanism which
breaks legacy hierarchies. Conditionalize instead of removing.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Tejun Heo
Jens Axboe
parent
c2aa723a60
commit
97c9341f72
51
mm/vmscan.c
51
mm/vmscan.c
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@ -154,11 +154,42 @@ static bool global_reclaim(struct scan_control *sc)
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{
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return !sc->target_mem_cgroup;
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}
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/**
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* sane_reclaim - is the usual dirty throttling mechanism operational?
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* @sc: scan_control in question
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*
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* The normal page dirty throttling mechanism in balance_dirty_pages() is
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* completely broken with the legacy memcg and direct stalling in
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* shrink_page_list() is used for throttling instead, which lacks all the
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* niceties such as fairness, adaptive pausing, bandwidth proportional
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* allocation and configurability.
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*
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* This function tests whether the vmscan currently in progress can assume
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* that the normal dirty throttling mechanism is operational.
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*/
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static bool sane_reclaim(struct scan_control *sc)
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{
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struct mem_cgroup *memcg = sc->target_mem_cgroup;
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if (!memcg)
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return true;
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#ifdef CONFIG_CGROUP_WRITEBACK
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if (cgroup_on_dfl(mem_cgroup_css(memcg)->cgroup))
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return true;
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#endif
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return false;
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}
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#else
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static bool global_reclaim(struct scan_control *sc)
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{
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return true;
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}
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static bool sane_reclaim(struct scan_control *sc)
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{
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return true;
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}
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#endif
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static unsigned long zone_reclaimable_pages(struct zone *zone)
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@ -941,10 +972,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
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* note that the LRU is being scanned too quickly and the
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* caller can stall after page list has been processed.
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*
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* 2) Global reclaim encounters a page, memcg encounters a
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* page that is not marked for immediate reclaim or
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* the caller does not have __GFP_IO. In this case mark
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* the page for immediate reclaim and continue scanning.
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* 2) Global or new memcg reclaim encounters a page that is
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* not marked for immediate reclaim or the caller does not
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* have __GFP_IO. In this case mark the page for immediate
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* reclaim and continue scanning.
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*
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* __GFP_IO is checked because a loop driver thread might
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* enter reclaim, and deadlock if it waits on a page for
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@ -958,7 +989,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
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* grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
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* may_enter_fs here is liable to OOM on them.
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*
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* 3) memcg encounters a page that is not already marked
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* 3) Legacy memcg encounters a page that is not already marked
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* PageReclaim. memcg does not have any dirty pages
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* throttling so we could easily OOM just because too many
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* pages are in writeback and there is nothing else to
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@ -973,7 +1004,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
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goto keep_locked;
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/* Case 2 above */
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} else if (global_reclaim(sc) ||
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} else if (sane_reclaim(sc) ||
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!PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
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/*
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* This is slightly racy - end_page_writeback()
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@ -1422,7 +1453,7 @@ static int too_many_isolated(struct zone *zone, int file,
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if (current_is_kswapd())
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return 0;
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if (!global_reclaim(sc))
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if (!sane_reclaim(sc))
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return 0;
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if (file) {
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@ -1614,10 +1645,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
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set_bit(ZONE_WRITEBACK, &zone->flags);
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/*
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* memcg will stall in page writeback so only consider forcibly
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* stalling for global reclaim
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* Legacy memcg will stall in page writeback so avoid forcibly
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* stalling here.
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*/
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if (global_reclaim(sc)) {
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if (sane_reclaim(sc)) {
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/*
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* Tag a zone as congested if all the dirty pages scanned were
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* backed by a congested BDI and wait_iff_congested will stall.
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