Messages in this thread | | | Subject | Re: [PATCH 4/4] iommu/arm-smmu-v3: Remove cmpxchg() in arm_smmu_cmdq_issue_cmdlist() | From | John Garry <> | Date | Thu, 16 Jul 2020 11:26:29 +0100 |
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On 16/07/2020 11:20, Will Deacon wrote: > 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@huawei.com> >> --- >> drivers/iommu/arm-smmu-v3.c | 101 ++++++++++++++++++++++-------------- >> 1 file changed, 61 insertions(+), 40 deletions(-) > > I must admit, you made me smile putting trivial@kernel.org on cc for this ;) >
Yes, quite ironic :)
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