I haven't gone through the whole email-chain, so, I might be asking
some repetitive questions. I will go through the email-chain later.
On Wed, May 20, 2020 at 7:37 AM Chris Down <chris@xxxxxxxxxxxxxx> wrote:
>
> In Facebook production, we've seen cases where cgroups have been put
> into allocator throttling even when they appear to have a lot of slack
> file caches which should be trivially reclaimable.
>
> Looking more closely, the problem is that we only try a single cgroup
> reclaim walk for each return to usermode before calculating whether or
> not we should throttle. This single attempt doesn't produce enough
> pressure to shrink for cgroups with a rapidly growing amount of file
> caches prior to entering allocator throttling.
In my experience it is usually shrink_slab which requires hammering
multiple times to actually reclaim memory.
>
> As an example, we see that threads in an affected cgroup are stuck in
> allocator throttling:
>
> # for i in $(cat cgroup.threads); do
> > grep over_high "/proc/$i/stack"
> > done
> [<0>] mem_cgroup_handle_over_high+0x10b/0x150
> [<0>] mem_cgroup_handle_over_high+0x10b/0x150
> [<0>] mem_cgroup_handle_over_high+0x10b/0x150
>
> ...however, there is no I/O pressure reported by PSI, despite a lot of
> slack file pages:
>
> # cat memory.pressure
> some avg10=78.50 avg60=84.99 avg300=84.53 total=5702440903
> full avg10=78.50 avg60=84.99 avg300=84.53 total=5702116959
> # cat io.pressure
> some avg10=0.00 avg60=0.00 avg300=0.00 total=78051391
> full avg10=0.00 avg60=0.00 avg300=0.00 total=78049640
> # grep _file memory.stat
> inactive_file 1370939392
> active_file 661635072
>
> This patch changes the behaviour to retry reclaim either until the
> current task goes below the 10ms grace period, or we are making no
> reclaim progress at all. In the latter case, we enter reclaim throttling
> as before.
>
> To a user, there's no intuitive reason for the reclaim behaviour to
> differ from hitting memory.high as part of a new allocation, as opposed
> to hitting memory.high because someone lowered its value. As such this
> also brings an added benefit: it unifies the reclaim behaviour between
> the two.
What was the initial reason to have different behavior in the first place?
>
> There's precedent for this behaviour: we already do reclaim retries when
> writing to memory.{high,max}, in max reclaim, and in the page allocator
> itself.
>
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> ---
> mm/memcontrol.c | 28 +++++++++++++++++++++++-----
> 1 file changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2df9510b7d64..b040951ccd6b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -73,6 +73,7 @@ EXPORT_SYMBOL(memory_cgrp_subsys);
>
> struct mem_cgroup *root_mem_cgroup __read_mostly;
>
> +/* The number of times we should retry reclaim failures before giving up. */
> #define MEM_CGROUP_RECLAIM_RETRIES 5
>
> /* Socket memory accounting disabled? */
> @@ -2228,17 +2229,22 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
> return 0;
> }
>
> -static void reclaim_high(struct mem_cgroup *memcg,
> - unsigned int nr_pages,
> - gfp_t gfp_mask)
> +static unsigned long reclaim_high(struct mem_cgroup *memcg,
> + unsigned int nr_pages,
> + gfp_t gfp_mask)
> {
> + unsigned long nr_reclaimed = 0;
> +
> do {
> if (page_counter_read(&memcg->memory) <= READ_ONCE(memcg->high))
> continue;
> memcg_memory_event(memcg, MEMCG_HIGH);
> - try_to_free_mem_cgroup_pages(memcg, nr_pages, gfp_mask, true);
> + nr_reclaimed += try_to_free_mem_cgroup_pages(memcg, nr_pages,
> + gfp_mask, true);
> } while ((memcg = parent_mem_cgroup(memcg)) &&
> !mem_cgroup_is_root(memcg));
> +
> + return nr_reclaimed;
> }
>
> static void high_work_func(struct work_struct *work)
> @@ -2378,16 +2384,20 @@ void mem_cgroup_handle_over_high(void)
> {
> unsigned long penalty_jiffies;
> unsigned long pflags;
> + unsigned long nr_reclaimed;
> unsigned int nr_pages = current->memcg_nr_pages_over_high;
Is there any benefit to keep current->memcg_nr_pages_over_high after
this change? Why not just use SWAP_CLUSTER_MAX?
> + int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
> struct mem_cgroup *memcg;
>
> if (likely(!nr_pages))
> return;
>
> memcg = get_mem_cgroup_from_mm(current->mm);
> - reclaim_high(memcg, nr_pages, GFP_KERNEL);
> current->memcg_nr_pages_over_high = 0;
>
> +retry_reclaim:
> + nr_reclaimed = reclaim_high(memcg, nr_pages, GFP_KERNEL);
> +
> /*
> * memory.high is breached and reclaim is unable to keep up. Throttle
> * allocators proactively to slow down excessive growth.
> @@ -2403,6 +2413,14 @@ void mem_cgroup_handle_over_high(void)
> if (penalty_jiffies <= HZ / 100)
> goto out;
>
> + /*
> + * If reclaim is making forward progress but we're still over
> + * memory.high, we want to encourage that rather than doing allocator
> + * throttling.
> + */
> + if (nr_reclaimed || nr_retries--)
> + goto retry_reclaim;
> +
> /*
> * If we exit early, we're guaranteed to die (since
> * schedule_timeout_killable sets TASK_KILLABLE). This means we don't
> --
> 2.26.2
>
