2020년 8월 27일 (목) 오후 10:35, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>님이 작성: > > On Wed, Aug 26, 2020 at 02:12:44PM +0900, Joonsoo Kim wrote: > > > > And, it requires to break current code > > > > layering that order-0 page is always handled by the pcplist. I'd prefer > > > > to avoid it so this patch uses different way to skip CMA page allocation > > > > from the pcplist. > > > > > > Well it would be much simpler and won't affect most of allocations. Better than > > > flushing pcplists IMHO. > > > > Hmm...Still, I'd prefer my approach. > > I prefer the pcp bypass approach. It's simpler and it does not incur a > pcp drain/refill penalty. > > > There are two reasons. First, > > layering problem > > mentioned above. In rmqueue(), there is a code for MIGRATE_HIGHATOMIC. > > As the name shows, it's for high order atomic allocation. But, after > > skipping pcplist > > allocation as you suggested, we could get there with order 0 request. > > I guess your concern is that under some circumstances that a request that > passes a watermark check could fail due to a highatomic reserve and to > an extent this is true. However, in that case the system is already low > on memory depending on the allocation context, the pcp lists may get > flushed anyway. My concern is that non-highorder (order-0) allocation could pollute/use the MIGRATE_HIGHATOMIC pageblock. It's reserved for highorder atomic allocation so it's not good if an order-0 request could get there. It would cause more fragmentation on that pageblock. > > We can also > > change this code, but, I'd hope to maintain current layering. Second, > > a performance > > reason. After the flag for nocma is up, a burst of nocma allocation > > could come. After > > flushing the pcplist one times, we can use the free page on the > > pcplist as usual until > > the context is changed. > > It's not guaranteed because CMA pages could be freed between the nocma save > and restore triggering further drains due to a reschedule. Similarly, > a CMA allocation in parallel could refill with CMA pages on the per-cpu > list. While both cases are unlikely, it's more unpredictable than a > straight-forward pcp bypass. Agreed that it's unpredictable than the pcp bypass. But, as you said, those cases would be rare. > I don't really see it as a layering violation of the API because all > order-0 pages go through the PCP lists. The fact that order-0 is serviced > from the pcp list is an internal implementation detail, the API doesn't > care. What I mean is an internal implementation layering violation. We could make a rule even in internal implementation to make code simpler and maintainable. I guess that order-0 is serviced from the pcp list is one of those. Anyway, although I prefer my approach, I'm okay with using pcp bypass. Thanks.
