Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs | Date | Fri, 28 May 2021 12:21:20 +0200 |
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On 28.05.21 12:09, Mel Gorman wrote: > On Fri, May 28, 2021 at 11:52:53AM +0200, David Hildenbrand wrote: >>>> "Disable pcplists so that page isolation cannot race with freeing >>>> in a way that pages from isolated pageblock are left on pcplists." >>>> >>>> Guess we'd then want to move the draining before start_isolate_page_range() >>>> in alloc_contig_range(). >>>> >>> >>> Or instead of draining, validate the PFN range in alloc_contig_range >>> is within the same zone and if so, call zone_pcp_disable() before >>> start_isolate_page_range and enable after __alloc_contig_migrate_range. >>> >> >> We require the caller to only pass a range within a single zone, so that >> should be fine. >> >> The only ugly thing about zone_pcp_disable() is >> mutex_lock(&pcp_batch_high_lock) which would serialize all >> alloc_contig_range() and even with offline_pages(). >> > > True so it would have to be accessed if that is bad or not. If racing > against offline_pages, memory is potentially being offlined in the > target zone which may cause allocation failure. If racing with other > alloc_contig_range calls, the two callers are potentially racing to > isolate and allocate the same range. The arguement could be made that > alloc_contig_ranges should be serialised within one zone to improve the > allocation success rate at the potential cost of allocation latency.
We have 3 main users of alloc_contig_range():
1. CMA
CMA synchronizes allocation within a CMA area via the allocation bitmap. So parallel CMA is perfectly possible and avoids races by design.
2. alloc_contig_pages() / Gigantic pages
Gigantic page allocation could race with virtio-mem. CMA does not apply. Possible but unlikely to matter in practice. virtio-mem will retry later again.
3. virito-mem
A virtio-mem device only operates on its assigned memory region, so we cannot have alloc_contig_range() from different devices racing, even when within a single zone. It could only race with gigantic pages as CMA does not apply.
So serializing would mostly harm parallel CMA (possible and likely) and parallel virtio-mem operation (e.g., unplug memory of two virtio-mem devices -- unlikely but possible).
Memory offlining racing with CMA is not an issue (impossible). virtio-mem synchronizes with memory offlining via memory notifiers, there is only a tiny little race window that usually doesn't matter as virtio-mem is expected to usually triggers offlining itself, and not user space rancomly. Memory offlining can race with dynamic gigantic page allocation, wich is highly unreliable already.
I wonder if we could optimize locking in zone_pcp_disable() instead.
-- Thanks,
David / dhildenb
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