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

[Leizhen (ThunderTown)]

On 2020/12/9 19:22, John Garry wrote:
> 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@xxxxxxxxxx/
>>>
>>> Analyzed-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>>> Reported-by: Xiang Chen <chenxiang66@xxxxxxxxxxxxx>
>>> Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> 
> 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?

Oh, Sorry, think of function free_cpu_cached_iovas() as function free_iova_rcaches().

Reviewed-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>

> 
> Cheers,
> John
> 
>>
>>>         return can_insert;
>>>   }
>>>
>>
>> .
>>
> 
> 
> .
>