Re: [RFC PATCH 1/2] blk-mq: Convert request->csd to call_single_data_t and reposition it

[Jens Axboe]

On 5/11/23 2:58?AM, Leonardo Bras wrote:
> Currently, request->csd has type struct __call_single_data.
> 
> call_single_data_t is defined in include/linux/smp.h :
> 
> /* Use __aligned() to avoid to use 2 cache lines for 1 csd */
> typedef struct __call_single_data call_single_data_t
> 	__aligned(sizeof(struct __call_single_data));
> 
> As the comment above the typedef suggests, having this struct split between
> 2 cachelines causes the need to fetch / invalidate / bounce 2 cachelines
> instead of 1 when the cpu receiving the request gets to run the requested
> function. This is usually bad for performance, due to one extra memory
> access and 1 extra cacheline usage.
> 
> Changing request->csd was previously attempted in commit
> 966a967116e6 ("smp: Avoid using two cache lines for struct call_single_data")
> but at the time the union that contains csd was positioned near the top of
> struct request, only below a struct list_head, and this caused the issue of
> holes summing up 24 extra bytes in struct request.
> 
> The struct size was restored back to normal by
> commit 4ccafe032005 ("block: unalign call_single_data in struct request")
> but it caused the csd to be possibly split in 2 cachelines again.
> 
> As an example with a 64-bit machine with
> CONFIG_BLK_RQ_ALLOC_TIME=y
> CONFIG_BLK_WBT=y
> CONFIG_BLK_DEV_INTEGRITY=y
> CONFIG_BLK_INLINE_ENCRYPTION=y
> 
> Will output pahole with:
> struct request {
> [...]
> 	union {
> 		struct __call_single_data csd;           /*   240    32 */
> 		u64                fifo_time;            /*   240     8 */
> 	};                                               /*   240    32 */
> [...]
> }
> 
> At this config, and any cacheline size between 32 and 256, will cause csd
> to be split between 2 cachelines: csd->node (16 bytes) in the first
> cacheline, and csd->func (8 bytes) & csd->info (8 bytes) in the second.
> 
> During blk_mq_complete_send_ipi(), csd->func and csd->info are getting
> changed, and when it calls __smp_call_single_queue() csd->node will get
> changed.
> 
> On the cpu which got the request, csd->func and csd->info get read by
> __flush_smp_call_function_queue() and csd->node gets changed by
> csd_unlock(), meaning the two cachelines containing csd will get accessed.
> 
> To avoid this, it would be necessary to change request->csd back to
> csd_single_data_t, which may end up increasing the struct size.
> (In above example, it increased from 288 to 320 -> 32 bytes).
> 
> In order to keep the csd_single_data_t and avoid the struct's size
> increase, move request->csd to the end of the struct.
> The rationale of this strategy is that for cachelines >= 32 bytes, there
> will never be used an extra cacheline for struct request:
> 
> - If request->csd is 32-byte aligned, there is no change in the object.
> - If request->csd is not 32-byte aligned, and part of it is in a different
>   cacheline, the whole csd is moved to that cacheline.
> - If request->csd is not 32-byte aligned, but it's all contained in the
>   same cacheline (> 32 bytes), aligning it to 32 will just put it a few
>   bytes forward in this cacheline.
> 
> (In above example, the change kept the struct's size in 288 bytes).
> 
> Convert request->csd to csd_single_data_t and move it to the end of
> struct request, so csd is never split between cachelines and don't use any
> extra cachelines.

This is going the wrong way imho. It'd be nice to get struct request
down to 256 bytes at some point, and then this would get in the way. The
patch is also attempting to do two things at once, which is a bad idea.

Why not just rearrange it a bit so that we don't split a cacheline with
the csd?

-- 
Jens Axboe