6fddf325fa
[ Upstream commit eca7de7cdc382eb6e0d344c07b1449ed75f5b435 ]
Improve the average delay precision of getdelay tool to microsecond. When
using the getdelay tool, it is sometimes found that the average delay
except CPU is not 0, but display is 0, because the precison is too low.
For example, see delay average of SWAP below when using ZRAM.
print delayacct stats ON
PID 32915
CPU count real total virtual total delay total delay average
339202 2793871936 9233585504 7951112 0.000ms
IO count delay total delay average
41 419296904 10ms
SWAP count delay total delay average
242589 1045792384 0ms
This wrong display is misleading, so improve the millisecond precision of
the average delay to microsecond just like CPU. Then user would get more
accurate information of delay time.
Link: https://lkml.kernel.org/r/202302131408087983857@zte.com.cn
Signed-off-by: Wang Yong <wang.yong12@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: 3840cbe24cf0 ("sched: psi: fix bogus pressure spikes from aggregation race")
Signed-off-by: Sasha Levin <sashal@kernel.org>
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Delay accounting
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================
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Tasks encounter delays in execution when they wait
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for some kernel resource to become available e.g. a
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runnable task may wait for a free CPU to run on.
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The per-task delay accounting functionality measures
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the delays experienced by a task while
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a) waiting for a CPU (while being runnable)
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b) completion of synchronous block I/O initiated by the task
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c) swapping in pages
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d) memory reclaim
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e) thrashing
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f) direct compact
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g) write-protect copy
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and makes these statistics available to userspace through
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the taskstats interface.
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Such delays provide feedback for setting a task's cpu priority,
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io priority and rss limit values appropriately. Long delays for
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important tasks could be a trigger for raising its corresponding priority.
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The functionality, through its use of the taskstats interface, also provides
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delay statistics aggregated for all tasks (or threads) belonging to a
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thread group (corresponding to a traditional Unix process). This is a commonly
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needed aggregation that is more efficiently done by the kernel.
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Userspace utilities, particularly resource management applications, can also
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aggregate delay statistics into arbitrary groups. To enable this, delay
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statistics of a task are available both during its lifetime as well as on its
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exit, ensuring continuous and complete monitoring can be done.
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Interface
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---------
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Delay accounting uses the taskstats interface which is described
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in detail in a separate document in this directory. Taskstats returns a
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generic data structure to userspace corresponding to per-pid and per-tgid
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statistics. The delay accounting functionality populates specific fields of
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this structure. See
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include/uapi/linux/taskstats.h
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for a description of the fields pertaining to delay accounting.
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It will generally be in the form of counters returning the cumulative
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delay seen for cpu, sync block I/O, swapin, memory reclaim, thrash page
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cache, direct compact, write-protect copy etc.
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Taking the difference of two successive readings of a given
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counter (say cpu_delay_total) for a task will give the delay
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experienced by the task waiting for the corresponding resource
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in that interval.
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When a task exits, records containing the per-task statistics
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are sent to userspace without requiring a command. If it is the last exiting
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task of a thread group, the per-tgid statistics are also sent. More details
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are given in the taskstats interface description.
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The getdelays.c userspace utility in tools/accounting directory allows simple
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commands to be run and the corresponding delay statistics to be displayed. It
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also serves as an example of using the taskstats interface.
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Usage
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-----
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Compile the kernel with::
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CONFIG_TASK_DELAY_ACCT=y
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CONFIG_TASKSTATS=y
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Delay accounting is disabled by default at boot up.
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To enable, add::
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delayacct
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to the kernel boot options. The rest of the instructions below assume this has
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been done. Alternatively, use sysctl kernel.task_delayacct to switch the state
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at runtime. Note however that only tasks started after enabling it will have
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delayacct information.
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After the system has booted up, use a utility
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similar to getdelays.c to access the delays
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seen by a given task or a task group (tgid).
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The utility also allows a given command to be
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executed and the corresponding delays to be
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seen.
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General format of the getdelays command::
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getdelays [-dilv] [-t tgid] [-p pid]
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Get delays, since system boot, for pid 10::
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# ./getdelays -d -p 10
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(output similar to next case)
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Get sum of delays, since system boot, for all pids with tgid 5::
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# ./getdelays -d -t 5
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print delayacct stats ON
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TGID 5
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CPU count real total virtual total delay total delay average
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8 7000000 6872122 3382277 0.423ms
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IO count delay total delay average
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0 0 0.000ms
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SWAP count delay total delay average
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0 0 0.000ms
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RECLAIM count delay total delay average
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0 0 0.000ms
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THRASHING count delay total delay average
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0 0 0.000ms
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COMPACT count delay total delay average
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0 0 0.000ms
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WPCOPY count delay total delay average
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0 0 0.000ms
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Get IO accounting for pid 1, it works only with -p::
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# ./getdelays -i -p 1
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printing IO accounting
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linuxrc: read=65536, write=0, cancelled_write=0
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The above command can be used with -v to get more debug information.
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