Messages in this thread | | | Date | Mon, 25 Jan 2021 14:00:14 +0000 | From | Chris Down <> | Subject | Re: [PATCH] Revert "mm: memcontrol: avoid workload stalls when lowering memory.high" |
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Johannes Weiner writes: >This reverts commit 536d3bf261a2fc3b05b3e91e7eef7383443015cf, as it >can cause writers to memory.high to get stuck in the kernel forever, >performing page reclaim and consuming excessive amounts of CPU cycles. > >Before the patch, a write to memory.high would first put the new limit >in place for the workload, and then reclaim the requested delta. After >the patch, the kernel tries to reclaim the delta before putting the >new limit into place, in order to not overwhelm the workload with a >sudden, large excess over the limit. However, if reclaim is actively >racing with new allocations from the uncurbed workload, it can keep >the write() working inside the kernel indefinitely. > >This is causing problems in Facebook production. A privileged >system-level daemon that adjusts memory.high for various workloads >running on a host can get unexpectedly stuck in the kernel and >essentially turn into a sort of involuntary kswapd for one of the >workloads. We've observed that daemon busy-spin in a write() for >minutes at a time, neglecting its other duties on the system, and >expending privileged system resources on behalf of a workload. > >To remedy this, we have first considered changing the reclaim logic to >break out after a couple of loops - whether the workload has converged >to the new limit or not - and bound the write() call this way. >However, the root cause that inspired the sequence change in the first >place has been fixed through other means, and so a revert back to the >proven limit-setting sequence, also used by memory.max, is preferable. > >The sequence was changed to avoid extreme latencies in the workload >when the limit was lowered: the sudden, large excess created by the >limit lowering would erroneously trigger the penalty sleeping code >that is meant to throttle excessive growth from below. Allocating >threads could end up sleeping long after the write() had already >reclaimed the delta for which they were being punished. > >However, erroneous throttling also caused problems in other scenarios >at around the same time. This resulted in commit b3ff92916af3 ("mm, >memcg: reclaim more aggressively before high allocator throttling"), >included in the same release as the offending commit. When allocating >threads now encounter large excess caused by a racing write() to >memory.high, instead of entering punitive sleeps, they will simply be >tasked with helping reclaim down the excess, and will be held no >longer than it takes to accomplish that. This is in line with regular >limit enforcement - i.e. if the workload allocates up against or over >an otherwise unchanged limit from below. > >With the patch breaking userspace, and the root cause addressed by >other means already, revert it again. > >Fixes: 536d3bf261a2 ("mm: memcontrol: avoid workload stalls when lowering memory.high") >Cc: <stable@vger.kernel.org> # 5.8+ >Reported-by: Tejun Heo <tj@kernel.org> >Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Chris Down <chris@chrisdown.name>
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