mm, fadvise: improve the expensive remote LRU cache draining after FADV_DONTNEED
Our users reported that there're some random latency spikes when their RT process is running. Finally we found that latency spike is caused by FADV_DONTNEED. Which may call lru_add_drain_all() to drain LRU cache on remote CPUs, and then waits the per-cpu work to complete. The wait time is uncertain, which may be tens millisecond. That behavior is unreasonable, because this process is bound to a specific CPU and the file is only accessed by itself, IOW, there should be no pagecache pages on a per-cpu pagevec of a remote CPU. That unreasonable behavior is partially caused by the wrong comparation of the number of invalidated pages and the number of the target. For example, if (count < (end_index - start_index + 1)) The count above is how many pages were invalidated in the local CPU, and (end_index - start_index + 1) is how many pages should be invalidated. The usage of (end_index - start_index + 1) is incorrect, because they are virtual addresses, which may not mapped to pages. Besides that, there may be holes between start and end. So we'd better check whether there are still pages on per-cpu pagevec after drain the local cpu, and then decide whether or not to call lru_add_drain_all(). After I applied it with a hotfix to our production environment, most of the lru_add_drain_all() can be avoided. Suggested-by: NMel Gorman <mgorman@suse.de> Signed-off-by: NYafang Shao <laoar.shao@gmail.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Acked-by: NMel Gorman <mgorman@suse.de> Cc: Johannes Weiner <hannes@cmpxchg.org> Link: https://lkml.kernel.org/r/20200923133318.14373-1-laoar.shao@gmail.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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