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    SubjectRe: [RFC PATCH 7/7] lazy tlb: shoot lazies, a non-refcounting lazy tlb option
    Excerpts from Nicholas Piggin's message of July 14, 2020 3:04 pm:
    > Excerpts from Andy Lutomirski's message of July 14, 2020 4:18 am:
    >>
    >>> On Jul 13, 2020, at 9:48 AM, Nicholas Piggin <npiggin@gmail.com> wrote:
    >>>
    >>> Excerpts from Andy Lutomirski's message of July 14, 2020 1:59 am:
    >>>>> On Thu, Jul 9, 2020 at 6:57 PM Nicholas Piggin <npiggin@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.
    >>>>>
    >>>>> On a 16-socket 192-core POWER8 system, a context switching benchmark
    >>>>> with as many software threads as CPUs (so each switch will go in and
    >>>>> out of idle), upstream can achieve a rate of about 1 million context
    >>>>> switches per second. After this patch it goes up to 118 million.
    >>>>>
    >>>>
    >>>> I read the patch a couple of times, and I have a suggestion that could
    >>>> be nonsense. You are, effectively, using mm_cpumask() as a sort of
    >>>> refcount. You're saying "hey, this mm has no more references, but it
    >>>> still has nonempty mm_cpumask(), so let's send an IPI and shoot down
    >>>> those references too." I'm wondering whether you actually need the
    >>>> IPI. What if, instead, you actually treated mm_cpumask as a refcount
    >>>> for real? Roughly, in __mmdrop(), you would only free the page tables
    >>>> if mm_cpumask() is empty. And, in the code that removes a CPU from
    >>>> mm_cpumask(), you would check if mm_users == 0 and, if so, check if
    >>>> you just removed the last bit from mm_cpumask and potentially free the
    >>>> mm.
    >>>>
    >>>> Getting the locking right here could be a bit tricky -- you need to
    >>>> avoid two CPUs simultaneously exiting lazy TLB and thinking they
    >>>> should free the mm, and you also need to avoid an mm with mm_users
    >>>> hitting zero concurrently with the last remote CPU using it lazily
    >>>> exiting lazy TLB. Perhaps this could be resolved by having mm_count
    >>>> == 1 mean "mm_cpumask() is might contain bits and, if so, it owns the
    >>>> mm" and mm_count == 0 meaning "now it's dead" and using some careful
    >>>> cmpxchg or dec_return to make sure that only one CPU frees it.
    >>>>
    >>>> Or maybe you'd need a lock or RCU for this, but the idea would be to
    >>>> only ever take the lock after mm_users goes to zero.
    >>>
    >>> I don't think it's nonsense, it could be a good way to avoid IPIs.
    >>>
    >>> I haven't seen much problem here that made me too concerned about IPIs
    >>> yet, so I think the simple patch may be good enough to start with
    >>> for powerpc. I'm looking at avoiding/reducing the IPIs by combining the
    >>> unlazying with the exit TLB flush without doing anything fancy with
    >>> ref counting, but we'll see.
    >>
    >> I would be cautious with benchmarking here. I would expect that the
    >> nasty cases may affect power consumption more than performance — the
    >> specific issue is IPIs hitting idle cores, and the main effects are to
    >> slow down exit() a bit but also to kick the idle core out of idle.
    >> Although, if the idle core is in a deep sleep, that IPI could be
    >> *very* slow.
    >
    > It will tend to be self-limiting to some degree (deeper idle cores
    > would tend to have less chance of IPI) but we have bigger issues on
    > powerpc with that, like broadcast IPIs to the mm cpumask for THP
    > management. Power hasn't really shown up as an issue but powerpc
    > CPUs may have their own requirements and issues there, shall we say.
    >
    >> So I think it’s worth at least giving this a try.
    >
    > To be clear it's not a complete solution itself. The problem is of
    > course that mm cpumask gives you false negatives, so the bits
    > won't always clean up after themselves as CPUs switch away from their
    > lazy tlb mms.

    ^^

    False positives: CPU is in the mm_cpumask, but is not using the mm
    as a lazy tlb. So there can be bits left and never freed.

    If you closed the false positives, you're back to a shared mm cache
    line on lazy mm context switches.

    Thanks,
    Nick

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