Re: [PATCH -V3] swap: Reduce lock contention on swap cache from swap slots allocation

[Huang\, Ying]

Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:

> On Mon, May 25, 2020 at 08:26:48AM +0800, Huang Ying wrote:
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 423c234aca15..0abd93d2a4fc 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -615,7 +615,8 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>>  			 * discarding, do discard now and reclaim them
>>  			 */
>>  			swap_do_scheduled_discard(si);
>> -			*scan_base = *offset = si->cluster_next;
>> +			*scan_base = this_cpu_read(*si->cluster_next_cpu);
>> +			*offset = *scan_base;
>>  			goto new_cluster;
>
> Why is this done?  As far as I can tell, the values always get overwritten at
> the end of the function with tmp and tmp isn't derived from them.  Seems
> ebc2a1a69111 moved some logic that used to make sense but doesn't have any
> effect now.

If we fail to allocate from cluster, "scan_base" and "offset" will not
be overridden.  And "cluster_next" or "cluster_next_cpu" may be changed
in swap_do_scheduled_discard(), because the lock is released and
re-acquired there.

The code may not have much value.  And you may think that it's better to
remove it.  But that should be in another patch.

>>  		} else
>>  			return false;
>> @@ -721,6 +722,34 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
>>  	}
>>  }
>>  
>> +static void set_cluster_next(struct swap_info_struct *si, unsigned long next)
>> +{
>> +	unsigned long prev;
>> +
>> +	if (!(si->flags & SWP_SOLIDSTATE)) {
>> +		si->cluster_next = next;
>> +		return;
>> +	}
>> +
>> +	prev = this_cpu_read(*si->cluster_next_cpu);
>> +	/*
>> +	 * Cross the swap address space size aligned trunk, choose
>> +	 * another trunk randomly to avoid lock contention on swap
>> +	 * address space if possible.
>> +	 */
>> +	if ((prev >> SWAP_ADDRESS_SPACE_SHIFT) !=
>> +	    (next >> SWAP_ADDRESS_SPACE_SHIFT)) {
>> +		/* No free swap slots available */
>> +		if (si->highest_bit <= si->lowest_bit)
>> +			return;
>> +		next = si->lowest_bit +
>> +			prandom_u32_max(si->highest_bit - si->lowest_bit + 1);
>> +		next = ALIGN(next, SWAP_ADDRESS_SPACE_PAGES);
>> +		next = max_t(unsigned int, next, si->lowest_bit);
>
> next is always greater than lowest_bit because it's aligned up.  I think the
> intent of the max_t line is to handle when next is aligned outside the valid
> range, so it'd have to be ALIGN_DOWN instead?

Oops.  I misunderstood "ALIGN()" here.  Yes.  we should use ALIGN_DOWN()
instead.  Thanks for pointing this out!

>
> These aside, patch looks good to me.

Thanks for your review!  It really help me to improve the quality of the
patch.  Can I add your "Reviewed-by" in the next version?

Best Regards,
Huang, Ying