Re: [RESEND PATCH v3 3/4] iommu/iova: Flush CPU rcache for when a depot fills

[John Garry <john.garry@huawei.com>]

On 09/12/2020 09:13, Leizhen (ThunderTown) wrote:
> 
> 
> On 2020/11/17 18:25, John Garry wrote:
>> Leizhen reported some time ago that IOVA performance may degrade over time
>> [0], but unfortunately his solution to fix this problem was not given
>> attention.
>>
>> To summarize, the issue is that as time goes by, the CPU rcache and depot
>> rcache continue to grow. As such, IOVA RB tree access time also continues
>> to grow.
>>
>> At a certain point, a depot may become full, and also some CPU rcaches may
>> also be full when inserting another IOVA is attempted. For this scenario,
>> currently the "loaded" CPU rcache is freed and a new one is created. This
>> freeing means that many IOVAs in the RB tree need to be freed, which
>> makes IO throughput performance fall off a cliff in some storage scenarios:
>>
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6314MB/0KB/0KB /s] [1616K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [5669MB/0KB/0KB /s] [1451K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6031MB/0KB/0KB /s] [1544K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6673MB/0KB/0KB /s] [1708K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6705MB/0KB/0KB /s] [1717K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6031MB/0KB/0KB /s] [1544K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6761MB/0KB/0KB /s] [1731K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6705MB/0KB/0KB /s] [1717K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6685MB/0KB/0KB /s] [1711K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6178MB/0KB/0KB /s] [1582K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6731MB/0KB/0KB /s] [1723K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [2387MB/0KB/0KB /s] [611K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [2689MB/0KB/0KB /s] [688K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [2278MB/0KB/0KB /s] [583K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [1288MB/0KB/0KB /s] [330K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [1632MB/0KB/0KB /s] [418K/0/0 iops]
>> Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [1765MB/0KB/0KB /s] [452K/0/0 iops]
>>
>> And continue in this fashion, without recovering. Note that in this
>> example it was required to wait 16 hours for this to occur. Also note that
>> IO throughput also becomes gradually becomes more unstable leading up to
>> this point.
>>
>> This problem is only seen for non-strict mode. For strict mode, the rcaches
>> stay quite compact.
>>
>> As a solution to this issue, judge that the IOVA caches have grown too big
>> when cached magazines need to be free, and just flush all the CPUs rcaches
>> instead.
>>
>> The depot rcaches, however, are not flushed, as they can be used to
>> immediately replenish active CPUs.
>>
>> In future, some IOVA compaction could be implemented to solve the
>> instabilty issue, which I figure could be quite complex to implement.
>>
>> [0] https://lore.kernel.org/linux-iommu/20190815121104.29140-3-thunder.leizhen@huawei.com/
>>
>> Analyzed-by: Zhen Lei <thunder.leizhen@huawei.com>
>> Reported-by: Xiang Chen <chenxiang66@hisilicon.com>
>> Signed-off-by: John Garry <john.garry@huawei.com>

Thanks for having a look

>> ---
>>   drivers/iommu/iova.c | 16 ++++++----------
>>   1 file changed, 6 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>> index 1f3f0f8b12e0..386005055aca 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -901,7 +901,6 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>>   				 struct iova_rcache *rcache,
>>   				 unsigned long iova_pfn)
>>   {
>> -	struct iova_magazine *mag_to_free = NULL;
>>   	struct iova_cpu_rcache *cpu_rcache;
>>   	bool can_insert = false;
>>   	unsigned long flags;
>> @@ -923,13 +922,12 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>>   				if (cpu_rcache->loaded)
>>   					rcache->depot[rcache->depot_size++] =
>>   							cpu_rcache->loaded;
>> -			} else {
>> -				mag_to_free = cpu_rcache->loaded;
>> +				can_insert = true;
>> +				cpu_rcache->loaded = new_mag;
>>   			}
>>   			spin_unlock(&rcache->lock);
>> -
>> -			cpu_rcache->loaded = new_mag;
>> -			can_insert = true;
>> +			if (!can_insert)
>> +				iova_magazine_free(new_mag);
>>   		}
>>   	}
>>   
>> @@ -938,10 +936,8 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>>   
>>   	spin_unlock_irqrestore(&cpu_rcache->lock, flags);
>>   
>> -	if (mag_to_free) {
>> -		iova_magazine_free_pfns(mag_to_free, iovad);
>> -		iova_magazine_free(mag_to_free);
> mag_to_free has been stripped out, that's why lock protection is not required here.
> 
>> -	}
>> +	if (!can_insert)
>> +		free_all_cpu_cached_iovas(iovad);
> Lock protection required.

But we have the per-CPU rcache locking again in free_cpu_cached_iovas() 
(which is called per-CPU from free_all_cpu_cached_iovas()).

ok? Or some other lock you mean?

Cheers,
John

> 
>>   
>>   	return can_insert;
>>   }
>>
> 
> .
> 

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