Re: [PATCH 2/7] x86,tlb: leave lazy TLB mode at page table free time
From: Rik van Riel
Date: Fri Jun 29 2018 - 12:56:33 EST
On Fri, 2018-06-29 at 09:39 -0700, Dave Hansen wrote:
> 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.
If we get to a v4, I will do that.
> > 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).
With this patch series, we never remove a CPU from
the mm_cpumask(mm) while in lazy TLB mode, but we
also do not send TLB shootdowns too CPUs in lazy TLB
mode, unless we are freeing page tables - when that
happens, the CPU will remove itself from the mm_cpumask.
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