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From
SubjectRe: [PATCH] 2.5.21 Nonlinear CPU support
Date
Followup to:  <5.1.0.14.2.20020612192802.045b08c0@pop.cus.cam.ac.uk>
By author: Anton Altaparmakov <aia21@cantab.net>
In newsgroup: linux.dev.kernel
>
> >2) If you need to do decompression on a cpu you check the array entry
> > for that CPU and if is NULL you vmalloc() the decompression buffers once
> > for that CPU. This avoid vmalloc() overhead for each read.
>
> The vmalloc() sleeps and by the time you get control back you are executing
> on a different CPU. Ooops. The only valid way of treating per-cpu data is:
>
> - disable preemption
> - get the cpu number = START OF CRITICAL SECTION: no sleep/schedule allowed
> - do work using the cpu number
> - reenable preemption = END OF CRITICAL SECTION
>

Actually, that doesn't matter, because it's a quickly convergent
operation.

Basically, once you've been invoked on a particular CPU once, you are
pretty much guaranteed to get invoked on that same CPU again, so the
fact that you may end up using a different buffer post-allocation is
not an issue.

Have an array and a semaphore called here allocation_semaphore:

/* PSEUDO-CODE */

while ( 1 ) {
disable_preemption();
cpu = current_cpu();
if ( decompression_buffers[cpu] ) {
do_decompression(decompression_buffers[cpu]);
enable_preemption();
break; /* DONE, EXIT LOOP */
} else {
enable_preemption();
down_sem(allocation_semaphore);
/* Avoid race condition here */
if ( !decompression_buffers[cpu] )
decompression_buffers[cpu] = vmalloc(BUFFER_SIZE);
up_sem(allocation_semaphore);
}
}

Note that there is no requirement that we're still on cpu "cpu" when
we allocate the buffer. Furthermore, if we fail, we just loop right
back to the top.

-hpa

--
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt <amsp@zytor.com>
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