Re: [PATCH bpf-next v5 4/7] memcg: Use trylock to access memcg stock_lock.

[Vlastimil Babka <vbabka@suse.cz>]

On 1/15/25 03:17, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Teach memcg to operate under trylock conditions when spinning locks
> cannot be used.
> 
> 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 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.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  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);

The last line can practially only happen on RT, right? On non-RT irqsave
means we could only fail the trylock from a nmi and then we should have
gfp_flags that don't allow spinning.

So suppose we used local_trylock(), local_lock() and local_unlock()  (no
_irqsave) instead, as I mentioned in reply to 3/7. The RT implementation
would be AFAICS the same. On !RT the trylock could now fail from a IRQ
context in addition to NMI context, but that should also have a gfp_mask
that does not allow spinning, so it should work fine.

It would however mean converting all users of the lock, i.e. also
consume_obj_stock() etc., but AFAIU that will be necessary anyway to have
opportunistic slab allocations?

> +	}
>  
>  	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))