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

[Roman Gushchin]

On Fri, Apr 09, 2021 at 07:18:42PM -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.

I agree, the extra memory space is not a significant concern.
I'd be more worried about the code complexity, but the result looks
nice to me!

Acked-by: Roman Gushchin <guro@xxxxxx>

Btw, it seems that the mm tree ran a bit off, so I had to apply this series
on top of Linus's tree to review. Please, rebase.

Thanks!