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

[Waiman Long <waiman.long@hp.com>]

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