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3624ba7b5e
After commit 8a99b6833c ("sched: Move SCHED_DEBUG sysctl to
debugfs"), some NUMA balancing sysctls enclosed with SCHED_DEBUG has
been moved to debugfs. This patch move the document for these
sysctls from
Documentation/admin-guide/sysctl/kernel.rst
to
Documentation/scheduler/sched-debug.rst
to make the document consistent with the code.
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: https://lkml.kernel.org/r/20220210052514.3038279-1-ying.huang@intel.com
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=================
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Scheduler debugfs
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=================
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Booting a kernel with CONFIG_SCHED_DEBUG=y will give access to
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scheduler specific debug files under /sys/kernel/debug/sched. Some of
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those files are described below.
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numa_balancing
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==============
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`numa_balancing` directory is used to hold files to control NUMA
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balancing feature. If the system overhead from the feature is too
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high then the rate the kernel samples for NUMA hinting faults may be
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controlled by the `scan_period_min_ms, scan_delay_ms,
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scan_period_max_ms, scan_size_mb` files.
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scan_period_min_ms, scan_delay_ms, scan_period_max_ms, scan_size_mb
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-------------------------------------------------------------------
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Automatic NUMA balancing scans tasks address space and unmaps pages to
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detect if pages are properly placed or if the data should be migrated to a
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memory node local to where the task is running. Every "scan delay" the task
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scans the next "scan size" number of pages in its address space. When the
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end of the address space is reached the scanner restarts from the beginning.
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In combination, the "scan delay" and "scan size" determine the scan rate.
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When "scan delay" decreases, the scan rate increases. The scan delay and
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hence the scan rate of every task is adaptive and depends on historical
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behaviour. If pages are properly placed then the scan delay increases,
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otherwise the scan delay decreases. The "scan size" is not adaptive but
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the higher the "scan size", the higher the scan rate.
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Higher scan rates incur higher system overhead as page faults must be
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trapped and potentially data must be migrated. However, the higher the scan
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rate, the more quickly a tasks memory is migrated to a local node if the
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workload pattern changes and minimises performance impact due to remote
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memory accesses. These files control the thresholds for scan delays and
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the number of pages scanned.
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``scan_period_min_ms`` is the minimum time in milliseconds to scan a
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tasks virtual memory. It effectively controls the maximum scanning
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rate for each task.
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``scan_delay_ms`` is the starting "scan delay" used for a task when it
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initially forks.
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``scan_period_max_ms`` is the maximum time in milliseconds to scan a
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tasks virtual memory. It effectively controls the minimum scanning
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rate for each task.
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``scan_size_mb`` is how many megabytes worth of pages are scanned for
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a given scan.
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