Re: [Xen-devel] [PATCHv5 1/3] rwlock: Add per-cpu reader-writer lock infrastructure

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jan 11, 2016 at 3:06 PM, Malcolm Crossley
<malcolm.crossley@xxxxxxxxxx> wrote:
> On 22/12/15 11:56, George Dunlap wrote:
>> On 18/12/15 16:08, Malcolm Crossley wrote:
>>> <snip>
>>> +
>>> +#ifndef NDEBUG
>>> +#define PERCPU_RW_LOCK_UNLOCKED(owner) { RW_LOCK_UNLOCKED, 0, owner }
>>> +static inline void _percpu_rwlock_owner_check(percpu_rwlock_t 
>>> **per_cpudata,
>>> +                                         percpu_rwlock_t *percpu_rwlock)
>>> +{
>>> +    ASSERT(per_cpudata == percpu_rwlock->percpu_owner);
>>> +}
>>> +#else
>>> +#define PERCPU_RW_LOCK_UNLOCKED(owner) { RW_LOCK_UNLOCKED, 0 }
>>> +#define _percpu_rwlock_owner_check(data, lock) ((void)0)
>>> +#endif
>>> +
>>> +#define DEFINE_PERCPU_RWLOCK_RESOURCE(l, owner) \
>>> +    percpu_rwlock_t l = PERCPU_RW_LOCK_UNLOCKED(&get_per_cpu_var(owner))
>>> +#define percpu_rwlock_resource_init(l, owner) \
>>> +    (*(l) = 
>>> (percpu_rwlock_t)PERCPU_RW_LOCK_UNLOCKED(&get_per_cpu_var(owner)))
>>> +
>>> +static inline void _percpu_read_lock(percpu_rwlock_t **per_cpudata,
>>> +                                         percpu_rwlock_t *percpu_rwlock)
>>
>> Is there a particular reason you chose to only use the "owner" value in
>> the struct to verify that the "per_cpudata" argument passed matched the
>> one you expected, rather than just getting rid of the "per_cpudata"
>> argument altogether and always using the pointer in the struct?
>
> Initially I was aiming to add percpu aspects to the rwlock without increasing
> the size of the rwlock structure itself, this was to keep data cache usage and
> memory allocations the same.
> It became clear that having a global writer_activating barrier would cause the
> read_lock to enter the slow path far too often. So I put the writer_activating
> variable in the percpu_rwlock_t, as writer_activating is just a bool then the
> additional data overhead should be small. Always having a 8 byte pointer may
> add a lot of overhead to data structures contain multiple rwlocks and thus
> cause additional allocation overhead.
>>
>> (i.e., _percpu_read_lock(percpu_rwlock_t *percpu_rwlock) { ...
>> per_cpudata = percpu_rwlock->percpu_owner; ... })
>>
>> I'm not an expert in this sort of micro-optimization, but it seems like
>> you're trading off storing a pointer in your rwlock struct for storing a
>> pointer at every call site.  Since you have to read writer_activating
>> for every lock or unlock anyway,
>
> writer_activating is not read on the read_unlock path. As these are rwlocks
> then I'm assuming the read lock/unlock paths are more critical for 
> performance.
> So I'd prefer to not do a read of the percpu_rwlock structure if it's not
> required (i.e. on the read unlock path)
> Furthermore, the single byte for the writer_activating variable is likely
> to have been read into cache by accesses to other parts of the data structure
> near the percpu_rwlock_t. If we add additional 8 bytes to the percpu_rwlock_t
> then this may not happen and it may also adjust the cache line alignment 
> aswell.
>
>> it doesn't seem like you'd actually be
>> saving that many memory fetches; but having only one copy in the cache,
>> rather than one copy per call site, would on the whole reduce both the
>> cache footprint and the total memory used (if only by a few bytes).
>
> If you put the owner pointer in the percpu_rwlock_t then wouldn't you have
> a copy per instance of percpu_rwlock_t? Surely this would use more cache than
> the handful of call site references to a global variable.
>
>>
>> It also makes the code cleaner to have only one argument, rather than
>> two which must match; but since in all the places you use it you end up
>> using a wrapper to give you a single argument anyway, I don't think that
>> matters in this case.  (i.e., if there's a good reason for having it at
>> the call site instead if in the struct, I'm fine with this approach).
>
> If you agree with my reasoning for the cache overhead and performance of the
> read unlock path being better with passing the percpu_data as an argument then
> I propose we keep the patches as is.

Yeah, I think you convinced me:

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

Thanks!
 -George

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