Messages in this thread | | | Subject | Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity for managed interrupt | Date | Tue, 24 Dec 2019 11:20:25 +0000 | From | Marc Zyngier <> |
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On 2019-12-24 01:59, Ming Lei wrote: > On Mon, Dec 23, 2019 at 10:47:07AM +0000, Marc Zyngier wrote: >> On 2019-12-23 10:26, John Garry wrote: >> > > > > > I've also managed to trigger some of them now that I have >> > > > > access to >> > > > > > a decent box with nvme storage. >> > > > > >> > > > > I only have 2x NVMe SSDs when this occurs - I should not be >> > > > > hitting this... >> > > > > >> > > > > Out of curiosity, have you tried >> > > > > > with the SMMU disabled? I'm wondering whether we hit some >> > > > > livelock >> > > > > > condition on unmapping buffers... >> > > > > >> > > > > No, but I can give it a try. Doing that should lower the CPU >> > > > > usage, though, >> > > > > so maybe masks the issue - probably not. >> > > > >> > > > Lots of CPU lockup can is performance issue if there isn't >> > > > obvious bug. >> > > > >> > > > I am wondering if you may explain it a bit why enabling SMMU >> may >> > > > save >> > > > CPU a it? >> > > The other way around. mapping/unmapping IOVAs doesn't comes for >> > > free. >> > > I'm trying to find out whether the NVMe map/unmap patterns >> trigger >> > > something unexpected in the SMMU driver, but that's a very long >> > > shot. >> > >> > So I tested v5.5-rc3 with and without the SMMU enabled, and >> without >> > the SMMU enabled I don't get the lockup. >> >> OK, so my hunch wasn't completely off... At least we have something >> to look into. >> >> [...] >> >> > Obviously this is not conclusive, especially with such limited >> > testing - 5 minute runs each. The CPU load goes up when disabling >> the >> > SMMU, but that could be attributed to extra throughput (1183K -> >> > 1539K) loading. >> > >> > I do notice that since we complete the NVMe request in irq >> context, >> > we also do the DMA unmap, i.e. talk to the SMMU, in the same >> context, >> > which is less than ideal. >> >> It depends on how much overhead invalidating the TLB adds to the >> equation, but we should be able to do some tracing and find out. >> >> > I need to finish for the Christmas break today, so can't check >> this >> > much further ATM. >> >> No worries. May I suggest creating a new thread in the new year, >> maybe >> involving Robin and Will as well? > > Zhang Yi has observed the CPU lockup issue once when running heavy IO > on > single nvme drive, and please CC him if you have new patch to try.
On which architecture? John was indicating that this also happen on x86.
> Then looks the DMA unmap cost is too big on aarch64 if SMMU is > involved.
So far, we don't have any data suggesting that this is actually the case. Also, other workloads (such as networking) do not exhibit this behaviour, while being least as unmap-heavy as NVMe is.
If the cross-architecture aspect is confirmed, this points more into the direction of an interaction between the NVMe subsystem and the DMA API more than an architecture-specific problem.
Given that we have so far very little data, I'd hold off any conclusion.
M. -- Jazz is not dead. It just smells funny...
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