On Thu, 21 Jul 2011 09:58:24 +0200
Michal Hocko <mhocko@xxxxxxx> wrote:
> percpu_charge_mutex protects from multiple simultaneous per-cpu charge
> caches draining because we might end up having too many work items.
> At least this was the case until 26fe6168 (memcg: fix percpu cached
> charge draining frequency) when we introduced a more targeted draining
> for async mode.
> Now that also sync draining is targeted we can safely remove mutex
> because we will not send more work than the current number of CPUs.
> FLUSHING_CACHED_CHARGE protects from sending the same work multiple
> times and stock->nr_pages == 0 protects from pointless sending a work
> if there is obviously nothing to be done. This is of course racy but we
> can live with it as the race window is really small (we would have to
> see FLUSHING_CACHED_CHARGE cleared while nr_pages would be still
> non-zero).
> The only remaining place where we can race is synchronous mode when we
> rely on FLUSHING_CACHED_CHARGE test which might have been set by other
> drainer on the same group but we should wait in that case as well.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
A concern.
> ---
> mm/memcontrol.c | 12 ++----------
> 1 files changed, 2 insertions(+), 10 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8180cd9..9d49a12 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2065,7 +2065,6 @@ struct memcg_stock_pcp {
> #define FLUSHING_CACHED_CHARGE (0)
> };
> static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> -static DEFINE_MUTEX(percpu_charge_mutex);
>
> /*
> * Try to consume stocked charge on this cpu. If success, one page is consumed
> @@ -2166,7 +2165,8 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
>
> for_each_online_cpu(cpu) {
> struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> - if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> + if (root_mem == stock->cached &&
> + test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> flush_work(&stock->work);
Doesn't this new check handle hierarchy ?
css_is_ancestor() will be required if you do this check.
BTW, this change should be in other patch, I think.
Thanks,
-Kame
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