Re: [PATCH v3 5/5] mm/memcg: Optimize user context object stock access

Waiman Long
2021-04-15T19:06:37Z

On 4/15/21 2:53 PM, Johannes Weiner wrote:
> On Thu, Apr 15, 2021 at 02:16:17PM -0400, Waiman Long wrote:
>> On 4/15/21 1:53 PM, Johannes Weiner wrote:
>>> On Tue, Apr 13, 2021 at 09:20:27PM -0400, Waiman Long wrote:
>>>> Most kmem_cache_alloc() calls are from user context. With instrumentation
>>>> enabled, the measured amount of kmem_cache_alloc() calls from non-task
>>>> context was about 0.01% of the total.
>>>>
>>>> The irq disable/enable sequence used in this case to access content
>>>> from object stock is slow. To optimize for user context access, there
>>>> are now two object stocks for task context and interrupt context access
>>>> respectively.
>>>>
>>>> The task context object stock can be accessed after disabling preemption
>>>> which is cheap in non-preempt kernel. The interrupt context object stock
>>>> can only be accessed after disabling interrupt. User context code can
>>>> access interrupt object stock, but not vice versa.
>>>>
>>>> The mod_objcg_state() function is also modified to make sure that memcg
>>>> and lruvec stat updates are done with interrupted disabled.
>>>>
>>>> The downside of this change is that there are more data stored in local
>>>> object stocks and not reflected in the charge counter and the vmstat
>>>> arrays. However, this is a small price to pay for better performance.
>>>>
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>>> Acked-by: Roman Gushchin <guro@fb.com>
>>>> Reviewed-by: Shakeel Butt <shakeelb@google.com>
>>> This makes sense, and also explains the previous patch a bit
>>> better. But please merge those two.
>> The reason I broke it into two is so that the patches are individually
>> easier to review. I prefer to update the commit log of patch 4 to explain
>> why the obj_stock structure is introduced instead of merging the two.
> Well I did not find them easier to review separately.
>
>>>> @@ -2327,7 +2365,9 @@ static void drain_local_stock(struct work_struct *dummy)
>>>> local_irq_save(flags);
>>>> stock = this_cpu_ptr(&memcg_stock);
>>>> - drain_obj_stock(&stock->obj);
>>>> + drain_obj_stock(&stock->irq_obj);
>>>> + if (in_task())
>>>> + drain_obj_stock(&stock->task_obj);
>>>> drain_stock(stock);
>>>> clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
>>>> @@ -3183,7 +3223,7 @@ static inline void mod_objcg_state(struct obj_cgroup *objcg,
>>>> memcg = obj_cgroup_memcg(objcg);
>>>> if (pgdat)
>>>> lruvec = mem_cgroup_lruvec(memcg, pgdat);
>>>> - __mod_memcg_lruvec_state(memcg, lruvec, idx, nr);
>>>> + mod_memcg_lruvec_state(memcg, lruvec, idx, nr);
>>>> rcu_read_unlock();
>>> This is actually a bug introduced in the earlier patch, isn't it?
>>> Calling __mod_memcg_lruvec_state() without irqs disabled...
>>>
>> Not really, in patch 3, mod_objcg_state() is called only in the stock update
>> context where interrupt had already been disabled. But now, that is no
>> longer the case, that is why i need to update mod_objcg_state() to make sure
>> irq is disabled before updating vmstat data array.
> Oh, I see it now. Man, that's subtle. We've had several very hard to
> track down preemption bugs in those stats, because they manifest as
> counter imbalances and you have no idea if there is a leak somewhere.
>
> The convention for these functions is that the __ prefix indicates
> that preemption has been suitably disabled. Please always follow this
> convention, even if the semantic change is temporary.
I see. I will fix that in the next version.
>
> Btw, is there a reason why the stock caching isn't just part of
> mod_objcg_state()? Why does the user need to choose if they want the
> caching or not? It's not like we ask for this when charging, either.
>
Yes, I can revert it to make mod_objcg_state() to call
mod_obj_stock_state() internally instead of the other way around.

Cheers,
Longman