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SubjectRe: [patch 1/4] - Potential performance bottleneck for Linxu TCP

* David Miller <davem@davemloft.net> wrote:

> We can make explicitl preemption checks in the main loop of
> tcp_recvmsg(), and release the socket and run the backlog if
> need_resched() is TRUE.
>
> This is the simplest and most elegant solution to this problem.

yeah, i like this one. If the problem is "too long locked section", then
the most natural solution is to "break up the lock", not to "boost the
priority of the lock-holding task" (which is what the proposed patch
does).

[ Also note that "sprinkle the code with preempt_disable()" kind of
solutions, besides hurting interactivity, are also a pain to resolve
in something like PREEMPT_RT. (unlike say a spinlock,
preempt_disable() is quite opaque in what data structure it protects,
etc., making it hard to convert it to a preemptible primitive) ]

> The one suggested in your patch and paper are way overkill, there is
> no reason to solve a TCP specific problem inside of the generic
> scheduler.

agreed.

What we could also add is a /reverse/ mechanism to the scheduler: a task
could query whether it has just a small amount of time left in its
timeslice, and could in that case voluntarily drop its current lock and
yield, and thus give up its current timeslice and wait for a new, full
timeslice, instead of being forcibly preempted due to lack of timeslices
with a possibly critical lock still held.

But the suggested solution here, to "prolong the running of this task
just a little bit longer" only starts a perpetual arms race between
users of such a facility and other kernel subsystems. (besides not being
adequate anyway, there can always be /so/ long lock-hold times that the
scheduler would have no other option but to preempt the task)

Ingo
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