Re: [PATCH v2 3/2] memcg: punt high overage reclaim to return-to-userland path

[Vladimir Davydov <vdavydov@parallels.com>]

On Fri, Sep 04, 2015 at 05:00:11PM -0400, Tejun Heo wrote:
> Currently, try_charge() tries to reclaim memory synchronously when the
> high limit is breached; however, if the allocation doesn't have
> __GFP_WAIT, synchronous reclaim is skipped.  If a process performs
> only speculative allocations, it can blow way past the high limit.
> This is actually easily reproducible by simply doing "find /".
> slab/slub allocator tries speculative allocations first, so as long as
> there's memory which can be consumed without blocking, it can keep
> allocating memory regardless of the high limit.
> 
> This patch makes try_charge() always punt the over-high reclaim to the
> return-to-userland path.  If try_charge() detects that high limit is
> breached, it adds the overage to current->memcg_nr_pages_over_high and
> schedules execution of mem_cgroup_handle_over_high() which performs
> synchronous reclaim from the return-to-userland path.
> 
> As long as kernel doesn't have a run-away allocation spree, this
> should provide enough protection while making kmemcg behave more
> consistently.

Another good thing about such an approach is that it copes with prio
inversion. Currently, a task with small memory.high might issue
memory.high reclaim on kmem charge with a bunch of various locks held.
If a task with a big value of memory.high needs any of these locks,
it'll have to wait until the low prio task finishes reclaim and releases
the locks. By handing over reclaim to task_work whenever possible we
might avoid this issue and improve overall performance.

> 
> v2: - Switched to reclaiming only the overage caused by current rather
>       than the difference between usage and high as suggested by
>       Michal.
>     - Don't record the memcg which went over high limit.  This makes
>       exit path handling unnecessary.  Dropped.
>     - Drop mentions of avoiding high stack usage from description as
>       suggested by Vladimir.  max limit still triggers direct reclaim.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov@parallels.com>

Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>

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