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

From: Ingo Molnar
Date: Thu Nov 30 2006 - 01:59:02 EST

* David Miller <davem@xxxxxxxxxxxxx> wrote:

> This is why my suggestion is to preempt_disable() as soon as we grab
> the socket lock, [...]

independently of the issue at hand, in general the explicit use of
preempt_disable() in non-infrastructure code is quite a heavy tool. Its
effects are heavy and global: it disables /all/ preemption (even on
PREEMPT_RT). Furthermore, when preempt_disable() is used for per-CPU
data structures then [unlike for example to a spin-lock] the connection
between the 'data' and the 'lock' is not explicit - causing all kinds of
grief when trying to convert such code to a different preemption model.
(such as PREEMPT_RT :-)

So my plan is to remove all "open-coded" use of preempt_disable() [and
raw use of local_irq_save/restore] from the kernel and replace it with
some facility that connects data and lock. (Note that this will not
result in any actual changes on the instruction level because internally
every such facility still maps to preempt_disable() on non-PREEMPT_RT
kernels, so on non-PREEMPT_RT kernels such code will still be the same
as before.)

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