Messages in this thread | | | Subject | Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting | From | Rik van Riel <> | Date | Sun, 29 Jul 2018 12:55:05 -0400 |
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On Sun, 2018-07-29 at 08:29 -0700, Andy Lutomirski wrote: > > On Jul 29, 2018, at 5:11 AM, Rik van Riel <riel@surriel.com> wrote: > > > > > On Sat, 2018-07-28 at 21:21 -0700, Andy Lutomirski wrote: > > > On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@surriel.com> > > > wrote: > > > > Conditionally skip lazy TLB mm refcounting. When an > > > > architecture > > > > has > > > > CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is > > > > used in > > > > lazy TLB mode anywhere will get shot down from exit_mmap, and > > > > there > > > > in no need to incur the cache line bouncing overhead of > > > > refcounting > > > > a lazy TLB mm. > > > > > > Unless I've misunderstood something, this patch results in idle > > > tasks > > > whose active_mm has been freed still having active_mm pointing at > > > freed memory. > > > > Patch 9/10 is supposed to ensure that the lazy TLB CPUs get > > switched to init_mm before an mm is freed. No CPU should ever > > have its active_mm pointing at a freed mm. > > > > Your message made me re-read the code, and now I realize that > > leave_mm does not actually do that. > > > > Looking at the other callers of leave_mm, I might not be the > > only one surprised by that; xen_drop_mm_ref comes to mind. > > > > I guess I should some code to leave_mm to have it actually > > clear active_mm and call the conditional refcount drop helper > > function. > > > > Does that clear up the confusion? > > Kind of. But what’s the point of keeping active_mm? On architectures > that opt in to the new mode, there won’t be any code that cares about > it’s value. What’s the benefit of keeping it around? If you ifdef > it out, then it can’t possibly point to freed memory, and there’s > nothing to worry about.
I would like to get to that point, but in a way that does not leave the code too difficult to follow.
Getting rid of ->active_mm in context_switch() is straightforward, but I am not sure at all what to do about idle_task_exit() for example.
All the subtleties I ran into just with this phase of the code suggests (to me at least) that we should probably do this one step at a time.
I agree on the same end goal, though :)
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