Re: [Xen-devel] [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support for the qspinlock

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Apr 09, 2015 at 05:41:44PM -0400, Waiman Long wrote:
> >>+void __init __pv_init_lock_hash(void)
> >>+{
> >>+   int pv_hash_size = 4 * num_possible_cpus();
> >>+
> >>+   if (pv_hash_size<  (1U<<  LFSR_MIN_BITS))
> >>+           pv_hash_size = (1U<<  LFSR_MIN_BITS);
> >>+   /*
> >>+    * Allocate space from bootmem which should be page-size aligned
> >>+    * and hence cacheline aligned.
> >>+    */
> >>+   pv_lock_hash = alloc_large_system_hash("PV qspinlock",
> >>+                                          sizeof(struct pv_hash_bucket),
> >>+                                          pv_hash_size, 0, HASH_EARLY,
> >>+                                   &pv_lock_hash_bits, NULL,
> >>+                                          pv_hash_size, pv_hash_size);
> >     pv_taps = lfsr_taps(pv_lock_hash_bits);
> >
> 
> I don't understand what you meant here.

Let me explain (even though I propose taking all the LFSR stuff out).

pv_lock_hash_bit is a runtime variable, therefore it cannot compile time
evaluate the forest of if statements required to compute the taps value.

Therefore its best to compute the taps _once_, and this seems like a
good place to do so.

> >>+                           goto done;
> >>+                   }
> >>+           }
> >>+
> >>+           hash = lfsr(hash, pv_lock_hash_bits, 0);
> >Since pv_lock_hash_bits is a variable, you end up running through that
> >massive if() forest to find the corresponding tap every single time. It
> >cannot compile-time optimize it.
> 
> The minimum bits size is now 8. So unless the system has more than 64 vCPUs,
> it will get the right value in the first if statement.

Still, no reason to not pre-compute the taps value, its simple enough.



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