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SubjectRe: [PATCH 2/7] x86,tlb: leave lazy TLB mode at page table free time
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On 06/29/2018 07:29 AM, Rik van Riel wrote:
> The latter problem can be prevented in two ways. The first is to
> always send a TLB shootdown IPI to CPUs in lazy TLB mode, while
> the second one is to only send the TLB shootdown at page table
> freeing time.

I've read this a few times, and I keep having to remind myself why we
"always send a TLB shootdown IPI to CPUs in lazy TLB mode". It's not
strictly CPUs in lazy TLB mode, right? It's just the one that are in
lazy TLB mode _and_ using the mm from which we are freeing page tables.

If you revise these again, would it make sense to add a little blurb like:

CPUs in lazy TLB mode are using the "wrong" page tables,
generally from a process's mm while running true kernel code
like the idle task. This is just as problematic when freeing
page tables from that mm as a real non-lazy user of the page
tables would be.

> The second should result in fewer IPIs, since operationgs like
> mprotect and madvise are very common with some workloads, but
> do not involve page table freeing. Also, on munmap, batching
> of page table freeing covers much larger ranges of virtual
> memory than the batching of unmapped user pages.

Doesn't this also result in fewer IPIs because it *removes* the
processor from the mm_cpumask(mm) and won't send IPIs to it any more?
As it stood before, we'd IPI a lazy CPU over and over, but this way we
just do it once, switch to another mm, and never touch for this mm again
(unless that CPU becomes non-lazy and switches to that mm again).

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