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pre-migration wait time is the time that a task unnecessarily spends
on the runqueue of a CPU but doesn't get switched-in there. In terms
of tracepoints, it is the time between sched:sched_wakeup and
sched:sched_migrate_task.
Let's say a task woke up on CPU2, then it got migrated to CPU4 and
then it's switched-in to CPU4. So, here pre-migration wait time is
time that it was waiting on runqueue of CPU2 after it is woken up.
The general pattern for pre-migration to occur is:
sched:sched_wakeup
sched:sched_migrate_task
sched:sched_switch
The sched:sched_waking event is used to capture the wakeup time,
as it aligns with the existing code and only introduces a negligible
time difference.
pre-migrations are generally not useful and it increases migrations.
This metric would be helpful in testing patches mainly related to wakeup
and load-balancer code paths as better wakeup logic would choose an
optimal CPU where task would be switched-in and thereby reducing pre-
migrations.
The sample output(s) when -P or --pre-migrations is used:
=================
time cpu task name wait time sch delay run time pre-mig time
[tid/pid] (msec) (msec) (msec) (msec)
--------------- ------ ------------------------------ --------- --------- --------- ---------
38456.720806 [0001] schbench[28634/28574] 4.917 4.768 1.004 0.000
38456.720810 [0001] rcu_preempt[18] 3.919 0.003 0.004 0.000
38456.721800 [0006] schbench[28779/28574] 23.465 23.465 1.999 0.000
38456.722800 [0002] schbench[28773/28574] 60.371 60.237 3.955 60.197
38456.722806 [0001] schbench[28634/28574] 0.004 0.004 1.996 0.000
38456.722811 [0001] rcu_preempt[18] 1.996 0.005 0.005 0.000
38456.723800 [0000] schbench[28833/28574] 4.000 4.000 3.999 0.000
38456.723800 [0004] schbench[28762/28574] 42.951 42.839 3.999 39.867
38456.723802 [0007] schbench[28812/28574] 43.947 43.817 3.999 40.866
38456.723804 [0001] schbench[28587/28574] 7.935 7.822 0.993 0.000
Signed-off-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Link: https://lore.kernel.org/r/20241004170756.18064-1-vineethr@linux.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.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 reStructuredText 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.