Re: [PATCH bpf-next v5 06/34] bpf: prepare for memcg-based memory accounting for bpf maps

[Song Liu]

> On Nov 13, 2020, at 11:40 AM, Roman Gushchin <guro@xxxxxx> wrote:
> 
> On Fri, Nov 13, 2020 at 09:46:49AM -0800, Song Liu wrote:
>> 
>> 
>>> On Nov 12, 2020, at 2:15 PM, Roman Gushchin <guro@xxxxxx> wrote:
>> 
>> [...]
>> 
>>> 
>>> +#ifdef CONFIG_MEMCG_KMEM
>>> +static __always_inline int __bpf_map_update_elem(struct bpf_map *map, void *key,
>>> +						 void *value, u64 flags)
>>> +{
>>> +	struct mem_cgroup *old_memcg;
>>> +	bool in_interrupt;
>>> +	int ret;
>>> +
>>> +	/*
>>> +	 * If update from an interrupt context results in a memory allocation,
>>> +	 * the memory cgroup to charge can't be determined from the context
>>> +	 * of the current task. Instead, we charge the memory cgroup, which
>>> +	 * contained a process created the map.
>>> +	 */
>>> +	in_interrupt = in_interrupt();
>>> +	if (in_interrupt)
>>> +		old_memcg = set_active_memcg(map->memcg);
>> 
>> set_active_memcg() checks in_interrupt() again. Maybe we can introduce another
>> helper to avoid checking it twice? Something like
>> 
>> static inline struct mem_cgroup *
>> set_active_memcg_int(struct mem_cgroup *memcg)
>> {
>>        struct mem_cgroup *old;
>> 
>>        old = this_cpu_read(int_active_memcg);
>>        this_cpu_write(int_active_memcg, memcg);
>>        return old;
>> }
> 
> Yeah, it's a good idea!
> 
> in_interrupt() check is very cheap (like checking some bits in a per-cpu variable),
> so I don't think there will be any measurable difference. So I suggest to implement
> it later as an enhancement on top (maybe in the next merge window), to avoid an another
> delay. Otherwise I'll need to send a patch to mm@, wait for reviews and an inclusion
> into the mm tree, etc). Does it work for you?

Yeah, that works. 

Acked-by: Song Liu <songliubraving@xxxxxx>