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Message-ID: <YAHRwZXoRZFJkgE8@larix.localdomain>
Date: Fri, 15 Jan 2021 18:32:49 +0100
From: Jean-Philippe Brucker <jean-philippe@...aro.org>
To: John Garry <john.garry@...wei.com>
Cc: joro@...tes.org, will@...nel.org, linux-kernel@...r.kernel.org,
linuxarm@...wei.com, iommu@...ts.linux-foundation.org,
robin.murphy@....com
Subject: Re: [PATCH v4 3/3] iommu/iova: Flush CPU rcache for when a depot
fills
On Thu, Dec 10, 2020 at 02:23:09AM +0800, 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.
It would be good to understand why the rcache doesn't stabilize. Could be
a bug, or just need some tuning
In strict mode, if a driver does Alloc-Free-Alloc and the first alloc
misses the rcache, the second allocation hits it. The same sequence in
non-strict mode misses the cache twice, because the IOVA is added to the
flush queue on Free.
So rather than AFAFAF.. we get AAA..FFF.., only once the fq_timer triggers
or the FQ is full. Interestingly the FQ size is 2x IOVA_MAG_SIZE, so we
could allocate 2 magazines worth of fresh IOVAs before alloc starts
hitting the cache. If a job allocates more than that, some magazines are
going to the depot, and with multi-CPU jobs those will get used on other
CPUs during the next alloc bursts, causing the progressive increase in
rcache consumption. I wonder if setting IOVA_MAG_SIZE > IOVA_FQ_SIZE helps
reuse of IOVAs?
Then again I haven't worked out the details, might be entirely wrong. I'll
have another look next week.
Thanks,
Jean
> 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
> instability 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@...wei.com>
> Reported-by: Xiang Chen <chenxiang66@...ilicon.com>
> Tested-by: Xiang Chen <chenxiang66@...ilicon.com>
> Signed-off-by: John Garry <john.garry@...wei.com>
> Reviewed-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
> 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 732ee687e0e2..39b7488de8bb 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -841,7 +841,6 @@ 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;
> @@ -863,13 +862,12 @@ 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);
> }
> }
>
> @@ -878,10 +876,8 @@ 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);
> - }
> + if (!can_insert)
> + free_all_cpu_cached_iovas(iovad);
>
> return can_insert;
> }
> --
> 2.26.2
>
> _______________________________________________
> iommu mailing list
> iommu@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu
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