Re: [PATCH 11/11] mm/page_alloc: Embed per_cpu_pages locking within the per-cpu structure

[Vlastimil Babka <vbabka@suse.cz>]

On 4/14/21 3:39 PM, Mel Gorman wrote:
> struct per_cpu_pages is protected by the pagesets lock but it can be
> embedded within struct per_cpu_pages at a minor cost. This is possible
> because per-cpu lookups are based on offsets. Paraphrasing an explanation
> from Peter Ziljstra
> 
>   The whole thing relies on:
> 
>     &per_cpu_ptr(msblk->stream, cpu)->lock == per_cpu_ptr(&msblk->stream->lock, cpu)
> 
>   Which is true because the lhs:
> 
>     (local_lock_t *)((zone->per_cpu_pages + per_cpu_offset(cpu)) + offsetof(struct per_cpu_pages, lock))
> 
>   and the rhs:
> 
>     (local_lock_t *)((zone->per_cpu_pages + offsetof(struct per_cpu_pages, lock)) + per_cpu_offset(cpu))
> 
>   are identical, because addition is associative.
> 
> More details are included in mmzone.h. This embedding is not completely
> free for three reasons.
> 
> 1. As local_lock does not return a per-cpu structure, the PCP has to
>    be looked up twice -- first to acquire the lock and again to get the
>    PCP pointer.
> 
> 2. For PREEMPT_RT and CONFIG_DEBUG_LOCK_ALLOC, local_lock is potentially
>    a spinlock or has lock-specific tracking. In both cases, it becomes
>    necessary to release/acquire different locks when freeing a list of
>    pages in free_unref_page_list.

Looks like this pattern could benefit from a local_lock API helper that would do
the right thing? It probably couldn't optimize much the CONFIG_PREEMPT_RT case
which would need to be unlock/lock in any case, but CONFIG_DEBUG_LOCK_ALLOC
could perhaps just keep the IRQ's disabled and just note the change of what's
acquired?

> 3. For most kernel configurations, local_lock_t is empty and no storage is
>    required. By embedding the lock, the memory consumption on PREEMPT_RT
>    and CONFIG_DEBUG_LOCK_ALLOC is higher.

But I wonder, is there really a benefit to this increased complexity? Before the
patch we had "pagesets" - a local_lock that protects all zones' pcplists. Now
each zone's pcplists have own local_lock. On !PREEMPT_RT we will never take the
locks of multiple zones from the same CPU in parallel, because we use
local_lock_irqsave(). Can that parallelism happen on PREEMPT_RT, because that
could perhaps justify the change?

> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---