linux-kernel - Re: [PATCH 4/4] iommu/arm-smmu-v3: Remove cmpxchg() in arm_smmu_cmdq_issue

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Date:   Thu, 16 Jul 2020 11:20:38 +0100
From:   Will Deacon <will@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     robin.murphy@....com, joro@...tes.org, trivial@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, maz@...nel.org
Subject: Re: [PATCH 4/4] iommu/arm-smmu-v3: Remove cmpxchg() in
 arm_smmu_cmdq_issue_cmdlist()

On Tue, Jun 23, 2020 at 01:28:40AM +0800, John Garry wrote:
> It has been shown that the cmpxchg() for finding space in the cmdq can
> be a bottleneck:
> - for more CPUs contending the cmdq, the cmpxchg() will fail more often
> - since the software-maintained cons pointer is updated on the same 64b
>   memory region, the chance of cmpxchg() failure increases again
> 
> The cmpxchg() is removed as part of 2 related changes:
> 
> - Update prod and cmdq owner in a single atomic add operation. For this, we
>   count the prod and owner in separate regions in prod memory.
> 
>   As with simple binary counting, once the prod+wrap fields overflow, they
>   will zero. They should never overflow into "owner" region, and we zero
>   the non-owner, prod region for each owner. This maintains the prod
>   pointer.
> 
>   As for the "owner", we now count this value, instead of setting a flag.
>   Similar to before, once the owner has finished gathering, it will clear
>   a mask. As such, a CPU declares itself as the "owner" when it reads zero
>   for this region. This zeroing will also clear possible overflow in
>   wrap+prod region, above.
> 
>   The owner is now responsible for all cmdq locking to avoid possible
>   deadlock. The owner will lock the cmdq for all non-owers it has gathered
>   when they have space in the queue and have written their entries.
> 
> - Check for space in the cmdq after the prod pointer has been assigned.
> 
>   We don't bother checking for space in the cmdq before assigning the prod
>   pointer, as this would be racy.
> 
>   So since the prod pointer is updated unconditionally, it would be common
>   for no space to be available in the cmdq when prod is assigned - that
>   is, according the software-maintained prod and cons pointer. So now
>   it must be ensured that the entries are not yet written and not until
>   there is space.
> 
>   How the prod pointer is maintained also leads to a strange condition
>   where the prod pointer can wrap past the cons pointer. We can detect this
>   condition, and report no space here. However, a prod pointer progressed
>   twice past the cons pointer cannot be detected. But it can be ensured that
>   this that this scenario does not occur, as we limit the amount of
>   commands any CPU can issue at any given time, such that we cannot
>   progress prod pointer further.
> 
> Signed-off-by: John Garry <john.garry@...wei.com>
> ---
>  drivers/iommu/arm-smmu-v3.c | 101 ++++++++++++++++++++++--------------
>  1 file changed, 61 insertions(+), 40 deletions(-)

I must admit, you made me smile putting trivial@...nel.org on cc for this ;)

Will