> On Jan 9, 2022, at 11:22 AM, Rik van Riel <riel@xxxxxxxxxxx> wrote: > > On Sun, 2022-01-09 at 00:49 -0800, Nadav Amit wrote: >> >> It is possible for instance to get rid of is_lazy, keep the CPU >> on mm_cpumask when switching to kernel thread, and then if/when >> an IPI is received remove it from cpumask to avoid further >> unnecessary TLB shootdown IPIs. >> >> I do not know whether it is a pure win, but there is a tradeoff. > > That's not a win at all. It is what we used to have before > the lazy mm stuff was re-introduced, and it led to quite a > few unnecessary IPIs, which are especially slow on virtual > machines, where the target CPU may not be running. You make a good point about VMs. IIUC Lazy-TLB serves several goals: 1. Avoid arch address-space switch to save switching time and TLB misses. 2. Prevent unnecessary IPIs while kernel threads run. 3. Avoid cache-contention on mm_cpumask when switching to a kernel thread. Your concern is with (2), and this one is easy to keep regardless of the rest. I am not sure that (3) is actually helpful, since it might lead to more cache activity than without lazy-TLB, but that is somewhat orthogonal to everything else. As for (1), which is the most fragile aspect, unless you use shadow page-tables, I am not sure there is a significant benefit.
