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

[Waiman Long <waiman.long@xxxxxx>]

On 04/13/2015 10:47 AM, Peter Zijlstra wrote:
> 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.
OK, I got it. That make sense.

> > > > +                               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.
Still, we need to keep the hash_bits value as it will needed by the 
hashing function.
I have taken out the lfsr code and use linear probing in the updated 
qspinlock patch that I am working on. However, we can always add that 
back in as an additional patch.
Cheers,
Longman

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