On Thu, Apr 15, 2021 at 04:53:46PM +0200, Vlastimil Babka wrote: > 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? > A helper could potentially be used but right now, there is only one call-site that needs this type of care so it may be overkill. A helper was proposed that can lookup and lock a per-cpu structure which is generally useful but does not suit the case where different locks need to be 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? > I don't think PREEMPT_RT gets additional parallelism because it's still a per-cpu structure that is being protected. The difference is whether we are protecting the CPU-N index for all per_cpu_pages or just one. The patch exists because it was asked why the lock was not embedded within the structure it's protecting. I initially thought that was unsafe and I was wrong as explained in the changelog. But now that I find it *can* be done but it's a bit ugly so I put it at the end of the series so it can be dropped if necessary. -- Mel Gorman SUSE Labs
