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7b167b6810
In our testing (camera recording), Miguel and Wei found unmap_page_range() takes above 6ms with preemption disabled easily. When I see that, the reason is it holds page table spinlock during entire 512 page operation in a PMD. 6.2ms is never trivial for user experince if RT task couldn't run in the time because it could make frame drop or glitch audio problem. I had a time to benchmark it via adding some trace_printk hooks between pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing device is 2018 premium mobile device. I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the task runs on little core even though it doesn't have any IPI and LRU lock contention. It's already too heavy. If I remove activate_page, 35-40% overhead of zap_pte_range is gone so most of overhead(about 0.7ms) comes from activate_page via mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could accumulate up to several ms. So this patch adds a check for need_resched() in the loop, and a preemption point if necessary. Link: http://lkml.kernel.org/r/20190731061440.GC155569@google.com Signed-off-by: Minchan Kim <minchan@kernel.org> Reported-by: Miguel de Dios <migueldedios@google.com> Reported-by: Wei Wang <wvw@google.com> Reviewed-by: Andrew Morton <akpm@linux-foundation.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.