Re: [PATCH bpf-next v4 4/6] memcg: Use trylock to access memcg stock_lock.

[Michal Hocko <mhocko@suse.com>]

On Mon 13-01-25 18:19:20, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Teach memcg to operate under trylock conditions when
> spinning locks cannot be used.
> The end result is __memcg_kmem_charge_page() and
> __memcg_kmem_uncharge_page() are safe to use from
> any context in RT and !RT.

> In !RT the NMI handler may fail to trylock stock_lock.
> In RT hard IRQ and NMI handlers will not attempt to trylock.

I believe this is local_trylock_irqsave specific thing that is not that
interesting for the particular code path. It is more useful to mention
consequences. I would phrase it this way.

local_trylock might fail and this would lead to charge cache bypass if
the calling context doesn't allow spinning (gfpflags_allow_spinning).
In those cases we try to charge the memcg counter directly and fail
early if that is not possible. This might cause a pre-mature charge
failing but it will allow an opportunistic charging that is safe from
try_alloc_pages path.
 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7b3503d12aaf..e4c7049465e0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1756,7 +1756,8 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
>   *
>   * returns true if successful, false otherwise.
>   */
> -static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> +static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
> +			  gfp_t gfp_mask)
>  {
>  	struct memcg_stock_pcp *stock;
>  	unsigned int stock_pages;
> @@ -1766,7 +1767,11 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  	if (nr_pages > MEMCG_CHARGE_BATCH)
>  		return ret;
>  
> -	local_lock_irqsave(&memcg_stock.stock_lock, flags);
> +	if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> +		if (!gfpflags_allow_spinning(gfp_mask))
> +			return ret;
> +		local_lock_irqsave(&memcg_stock.stock_lock, flags);
> +	}
>  
>  	stock = this_cpu_ptr(&memcg_stock);
>  	stock_pages = READ_ONCE(stock->nr_pages);
> @@ -1851,7 +1856,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  {
>  	unsigned long flags;
>  
> -	local_lock_irqsave(&memcg_stock.stock_lock, flags);
> +	if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> +		/*
> +		 * In case of unlikely failure to lock percpu stock_lock
> +		 * uncharge memcg directly.
> +		 */
> +		mem_cgroup_cancel_charge(memcg, nr_pages);
> +		return;
> +	}
>  	__refill_stock(memcg, nr_pages);
>  	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>  }
> @@ -2196,9 +2208,13 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	unsigned long pflags;
>  
>  retry:
> -	if (consume_stock(memcg, nr_pages))
> +	if (consume_stock(memcg, nr_pages, gfp_mask))
>  		return 0;
>  
> +	if (!gfpflags_allow_spinning(gfp_mask))
> +		/* Avoid the refill and flush of the older stock */
> +		batch = nr_pages;
> +
>  	if (!do_memsw_account() ||
>  	    page_counter_try_charge(&memcg->memsw, batch, &counter)) {
>  		if (page_counter_try_charge(&memcg->memory, batch, &counter))
> -- 
> 2.43.5

-- 
Michal Hocko
SUSE Labs