Re: [PATCH] 2.5.21 Nonlinear CPU support

["H. Peter Anvin" <hpa@zytor.com>]

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
      
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