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mm: vmscan: disable memcg direct reclaim stalling if cgroup writeback support is in use

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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 2015-05-22 18:23:36 -04:00 committed by Jens Axboe
parent c2aa723a60
commit 97c9341f72
1 changed files with 41 additions and 10 deletions
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51
mm/vmscan.c
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@ -154,11 +154,42 @@ static bool global_reclaim(struct scan_control *sc)
{
return !sc->target_mem_cgroup;
}
/**
* sane_reclaim - is the usual dirty throttling mechanism operational?
* @sc: scan_control in question
*
* The normal page dirty throttling mechanism in balance_dirty_pages() is
* completely broken with the legacy memcg and direct stalling in
* shrink_page_list() is used for throttling instead, which lacks all the
* niceties such as fairness, adaptive pausing, bandwidth proportional
* allocation and configurability.
*
* This function tests whether the vmscan currently in progress can assume
* that the normal dirty throttling mechanism is operational.
*/
static bool sane_reclaim(struct scan_control *sc)
{
struct mem_cgroup *memcg = sc->target_mem_cgroup;
if (!memcg)
return true;
#ifdef CONFIG_CGROUP_WRITEBACK
if (cgroup_on_dfl(mem_cgroup_css(memcg)->cgroup))
return true;
#endif
return false;
}
#else
static bool global_reclaim(struct scan_control *sc)
{
return true;
}
static bool sane_reclaim(struct scan_control *sc)
{
return true;
}
#endif
static unsigned long zone_reclaimable_pages(struct zone *zone)
@ -941,10 +972,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
* note that the LRU is being scanned too quickly and the
* caller can stall after page list has been processed.
*
* 2) Global reclaim encounters a page, memcg encounters a
* page that is not marked for immediate reclaim or
* the caller does not have __GFP_IO. In this case mark
* the page for immediate reclaim and continue scanning.
* 2) Global or new memcg reclaim encounters a page that is
* not marked for immediate reclaim or the caller does not
* have __GFP_IO. In this case mark the page for immediate
* reclaim and continue scanning.
*
* __GFP_IO is checked because a loop driver thread might
* enter reclaim, and deadlock if it waits on a page for
@ -958,7 +989,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
* grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
* may_enter_fs here is liable to OOM on them.
*
* 3) memcg encounters a page that is not already marked
* 3) Legacy memcg encounters a page that is not already marked
* PageReclaim. memcg does not have any dirty pages
* throttling so we could easily OOM just because too many
* pages are in writeback and there is nothing else to
@ -973,7 +1004,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
goto keep_locked;
/* Case 2 above */
} else if (global_reclaim(sc) ||
} else if (sane_reclaim(sc) ||
!PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
/*
* This is slightly racy - end_page_writeback()
@ -1422,7 +1453,7 @@ static int too_many_isolated(struct zone *zone, int file,
if (current_is_kswapd())
return 0;
if (!global_reclaim(sc))
if (!sane_reclaim(sc))
return 0;
if (file) {
@ -1614,10 +1645,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
set_bit(ZONE_WRITEBACK, &zone->flags);
/*
* memcg will stall in page writeback so only consider forcibly
* stalling for global reclaim
* Legacy memcg will stall in page writeback so avoid forcibly
* stalling here.
*/
if (global_reclaim(sc)) {
if (sane_reclaim(sc)) {
/*
* Tag a zone as congested if all the dirty pages scanned were
* backed by a congested BDI and wait_iff_congested will stall.