Re: [tip:x86/mm] x86/mm/tlb: Make lazy TLB mode lazier

From: Peter Zijlstra
Date: Tue Jul 17 2018 - 07:33:48 EST


On Tue, Jul 17, 2018 at 02:35:08AM -0700, tip-bot for Rik van Riel wrote:
> @@ -242,17 +244,40 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> next->context.ctx_id);
>
> /*
> + * Even in lazy TLB mode, the CPU should stay set in the
> + * mm_cpumask. The TLB shootdown code can figure out from
> + * from cpu_tlbstate.is_lazy whether or not to send an IPI.
> */
> if (WARN_ON_ONCE(real_prev != &init_mm &&
> !cpumask_test_cpu(cpu, mm_cpumask(next))))
> cpumask_set_cpu(cpu, mm_cpumask(next));
>
> + /*
> + * If the CPU is not in lazy TLB mode, we are just switching
> + * from one thread in a process to another thread in the same
> + * process. No TLB flush required.
> + */
> + if (!was_lazy)
> + return;
> +
> + /*
> + * Read the tlb_gen to check whether a flush is needed.
> + * If the TLB is up to date, just use it.
> + * The barrier synchronizes with the tlb_gen increment in
> + * the TLB shootdown code.
> + */
> + smp_mb();

What exactly is this smp_mb() ordering? The above comment is
insufficient. Is it the cpumask_set_cpu() vs the atomic64_read() ?

If so, should this not be smp_mb__after_atomic() (iow a NO-OP on x86)

If it is not so, please fix the comment to explain things.

> + next_tlb_gen = atomic64_read(&next->context.tlb_gen);
> + if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
> + next_tlb_gen)
> + return;
> +
> + /*
> + * TLB contents went out of date while we were in lazy
> + * mode. Fall through to the TLB switching code below.
> + */
> + new_asid = prev_asid;
> + need_flush = true;