Re: [PATCH] 2.5.21 Nonlinear CPU support

From: H. Peter Anvin (
Date: Wed Jun 12 2002 - 15:03:14 EST

Followup to: <>
By author: Anton Altaparmakov <>
In newsgroup:
> >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

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:


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

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.


<> at work, <> in private!
"Unix gives you enough rope to shoot yourself in the foot."	<>
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