Re: [PATCH v6] mm/zswap: move to use crypto_acomp API for hardware acceleration

[Sebastian Andrzej Siewior]

On 2020-09-29 05:14:31 [+0000], Song Bao Hua (Barry Song) wrote:
> After second thought and trying to make this change, I would like to change my mind
> and disagree with this idea. Two reasons:
> 1. while using this_cpu_ptr() without preemption lock, people usually put all things bound
> with one cpu to one structure, so that once we get the pointer of the whole structure, we get
> all its parts belonging to the same cpu. If we move the dstmem and mutex out of the structure
> containing them, we will have to do:
> 	a. get_cpu_ptr() for the acomp_ctx   //lock preemption
> 	b. this_cpu_ptr() for the dstmem and mutex
> 	c. put_cpu_ptr() for the acomp_ctx  //unlock preemption
> 	d. mutex_lock()
> 	  sg_init_one()
> 	  compress/decompress etc.
> 	  ...
> 	  mutex_unlock
> 
> as the get() and put() have a preemption lock/unlock, this will make certain this_cpu_ptr()
> in the step "b" will return the right dstmem and mutex which belong to the same cpu with
> step "a".
> 
> The steps from "a" to "c" are quite silly and confusing. I believe the existing code aligns
> with the most similar code in kernel better:
> 	a. this_cpu_ptr()   //get everything for one cpu
> 	b. mutex_lock()
> 	  sg_init_one()
> 	  compress/decompress etc.
> 	  ...
> 	  mutex_unlock

My point was that there will be a warning at run-time and you don't want
that. There are raw_ accessors if you know what you are doing. But…

Earlier you had compression/decompression with disabled preemption and
strict per-CPU memory allocation. Now if you keep this per-CPU memory
allocation then you gain a possible bottleneck.
In the previous email you said that there may be a bottleneck in the
upper layer where you can't utilize all that memory you allocate. So you
may want to rethink that strategy before that rework.

> 2. while allocating mutex, we can put the mutex into local memory by using kmalloc_node().
> If we move to "struct mutex lock" directly, most CPUs in a NUMA server will have to access
> remote memory to read/write the mutex, therefore, this will increase the latency dramatically.

If you need something per-CPU then DEFINE_PER_CPU() will give it to you.
It would be very bad for performance if this allocations were not from
CPU-local memory, right? So what makes you think this is worse than
kmalloc_node() based allocations?

> Thanks
> Barry

Sebastian