On Tue, May 4, 2021 at 12:25 PM Jesse Brandeburg <jesse.brandeburg@xxxxxxxxx> wrote: > > Robin Murphy wrote: > > > On 2021-05-01 03:18, Jesse Brandeburg wrote: > > > It was pointed out by Nitesh that the original work I did in 2014 > > > to automatically set the interrupt affinity when requesting a > > > mask is no longer necessary. The kernel has moved on and no > > > longer has the original problem, BUT the original patch > > > introduced a subtle bug when booting a system with reserved or > > > excluded CPUs. Drivers calling this function with a mask value > > > that included a CPU that was currently or in the future > > > unavailable would generally not update the hint. > > > > > > I'm sure there are a million ways to solve this, but the simplest > > > one is to just remove a little code that tries to force the > > > affinity, as Nitesh has shown it fixes the bug and doesn't seem > > > to introduce immediate side effects. > > > > Unfortunately, I think there are quite a few other drivers now relying > > on this behaviour, since they are really using irq_set_affinity_hint() > > as a proxy for irq_set_affinity(). Partly since the latter isn't > > exported to modules, but also I have a vague memory of it being said > > that it's nice to update the user-visible hint to match when the > > affinity does have to be forced to something specific. > > > > Robin. > > Thanks for your feedback Robin, but there is definitely a bug here that > is being exposed by this code. The fact that people are using this > function means they're all exposed to this bug. > > Not sure if you saw, but this analysis from Nitesh explains what > happened chronologically to the kernel w.r.t this code, it's a useful > analysis! [1] > > I'd add in addition that irqbalance daemon *stopped* paying attention > to hints quite a while ago, so I'm not quite sure what purpose they > serve. > > [1] > https://lore.kernel.org/lkml/CAFki+Lm0W_brLu31epqD3gAV+WNKOJfVDfX2M8ZM__aj3nv9uA@xxxxxxxxxxxxxx/ > Wanted to follow up to see if there are any more objections or even suggestions to take this forward? -- Thanks Nitesh
