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0a65bc27bdAvoid an edge case where epoll_wait arms a timer and calls schedule()
even if the timer will expire immediately.
For example: if the user has specified an epoll busy poll usecs which is
equal or larger than the epoll_wait/epoll_pwait2 timeout, it is
unnecessary to call schedule_hrtimeout_range; the busy poll usecs have
consumed the entire timeout duration so it is unnecessary to induce
scheduling latency by calling schedule() (via schedule_hrtimeout_range).
This can be measured using a simple bpftrace script:
tracepoint:sched:sched_switch
/ args->prev_pid == $1 /
{
print(kstack());
print(ustack());
}
Before this patch is applied:
Testing an epoll_wait app with busy poll usecs set to 1000, and
epoll_wait timeout set to 1ms using the script above shows:
__traceiter_sched_switch+69
__schedule+1495
schedule+32
schedule_hrtimeout_range+159
do_epoll_wait+1424
__x64_sys_epoll_wait+97
do_syscall_64+95
entry_SYSCALL_64_after_hwframe+118
epoll_wait+82
Which is unexpected; the busy poll usecs should have consumed the
entire timeout and there should be no reason to arm a timer.
After this patch is applied: the same test scenario does not generate a
call to schedule() in the above edge case. If the busy poll usecs are
reduced (for example usecs: 100, epoll_wait timeout 1ms) the timer is
armed as expected.
Fixes:
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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.