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SubjectRe: [PATCH v15 09/15] pvqspinlock: Implement simple paravirt support for the qspinlock
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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