[PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

[Alexander Gordeev]

On Mon, Nov 30, 2020 at 10:31:51AM -0800, Andy Lutomirski wrote:
> other arch folk: there's some background here:
> 
> https://lkml.kernel.org/r/CALCETrVXUbe8LfNn-Qs+DzrOQaiw+sFUg1J047yByV31SaTOZw@mail.gmail.com
> 
> On Sun, Nov 29, 2020 at 12:16 PM Andy Lutomirski <luto at kernel.org> wrote:
> >
> > On Sat, Nov 28, 2020 at 7:54 PM Andy Lutomirski <luto at kernel.org> wrote:
> > >
> > > On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin at gmail.com> wrote:
> > > >
> > > > On big systems, the mm refcount can become highly contented when doing
> > > > a lot of context switching with threaded applications (particularly
> > > > switching between the idle thread and an application thread).
> > > >
> > > > Abandoning lazy tlb slows switching down quite a bit in the important
> > > > user->idle->user cases, so so instead implement a non-refcounted scheme
> > > > that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down
> > > > any remaining lazy ones.
> > > >
> > > > Shootdown IPIs are some concern, but they have not been observed to be
> > > > a big problem with this scheme (the powerpc implementation generated
> > > > 314 additional interrupts on a 144 CPU system during a kernel compile).
> > > > There are a number of strategies that could be employed to reduce IPIs
> > > > if they turn out to be a problem for some workload.
> > >
> > > I'm still wondering whether we can do even better.
> > >
> >
> > Hold on a sec.. __mmput() unmaps VMAs, frees pagetables, and flushes
> > the TLB.  On x86, this will shoot down all lazies as long as even a
> > single pagetable was freed.  (Or at least it will if we don't have a
> > serious bug, but the code seems okay.  We'll hit pmd_free_tlb, which
> > sets tlb->freed_tables, which will trigger the IPI.)  So, on
> > architectures like x86, the shootdown approach should be free.  The
> > only way it ought to have any excess IPIs is if we have CPUs in
> > mm_cpumask() that don't need IPI to free pagetables, which could
> > happen on paravirt.
> 
> Indeed, on x86, we do this:
> 
> [   11.558844]  flush_tlb_mm_range.cold+0x18/0x1d
> [   11.559905]  tlb_finish_mmu+0x10e/0x1a0
> [   11.561068]  exit_mmap+0xc8/0x1a0
> [   11.561932]  mmput+0x29/0xd0
> [   11.562688]  do_exit+0x316/0xa90
> [   11.563588]  do_group_exit+0x34/0xb0
> [   11.564476]  __x64_sys_exit_group+0xf/0x10
> [   11.565512]  do_syscall_64+0x34/0x50
> 
> and we have info->freed_tables set.
> 
> What are the architectures that have large systems like?
> 
> x86: we already zap lazies, so it should cost basically nothing to do
> a little loop at the end of __mmput() to make sure that no lazies are
> left.  If we care about paravirt performance, we could implement one
> of the optimizations I mentioned above to fix up the refcounts instead
> of sending an IPI to any remaining lazies.
> 
> arm64: AFAICT arm64's flush uses magic arm64 hardware support for
> remote flushes, so any lazy mm references will still exist after
> exit_mmap().  (arm64 uses lazy TLB, right?)  So this is kind of like
> the x86 paravirt case.  Are there large enough arm64 systems that any
> of this matters?
> 
> s390x: The code has too many acronyms for me to understand it fully,
> but I think it's more or less the same situation as arm64.  How big do
> s390x systems come?
> 
> power: Ridiculously complicated, seems to vary by system and kernel config.
> 
> So, Nick, your unconditional IPI scheme is apparently a big
> improvement for power, and it should be an improvement and have low
> cost for x86.  On arm64 and s390x it will add more IPIs on process
> exit but reduce contention on context switching depending on how lazy

s390 does not invalidate TLBs per-CPU explicitly - we have special
instructions for that. Those in turn initiate signalling to other
CPUs, completely transparent to OS.

Apart from mm_count, I am struggling to realize how the suggested
scheme could change the the contention on s390 in connection with
TLB. Could you clarify a bit here, please?

> TLB works.  I suppose we could try it for all architectures without
> any further optimizations.  Or we could try one of the perhaps
> excessively clever improvements I linked above.  arm64, s390x people,
> what do you think?

I do not immediately see anything in the series that would harm
performance on s390.

We however use mm_cpumask to distinguish between local and global TLB
flushes. With this series it looks like mm_cpumask is *required* to
be consistent with lazy users. And that is something quite diffucult
for us to adhere (at least in the foreseeable future).

But actually keeping track of lazy users in a cpumask is something
the generic code would rather do AFAICT.

Thanks!