Re: [PATCH v3 0/4] shoot lazy tlbs

Nicholas Piggin
2021-06-05T05:05:19Z

Excerpts from Nicholas Piggin's message of June 5, 2021 10:17 am:
> Excerpts from Andy Lutomirski's message of June 5, 2021 3:05 am:
>> On 6/4/21 9:54 AM, Andy Lutomirski wrote:
>>> On 5/31/21 11:22 PM, Nicholas Piggin wrote:
>>>> There haven't been objections to the series since last posting, this
>>>> is just a rebase and tidies up a few comments minor patch rearranging.
>>>>
>>>
>>> I continue to object to having too many modes. I like my more generic
>>> improvements better. Let me try to find some time to email again.
>>>
>>
>> Specifically, this:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/mm
>
> That's worse than what powerpc does with the shoot lazies code so
> we wouldn't use it anyway.
>
> The fact is mm-cpumask and lazy mm is very architecture specific, so I
> don't really see that another "mode" is such a problem, it's for the
> most part "this is what powerpc does" -> "this is what powerpc does".
> The only mode in the context switch is just "take a ref on the lazy mm"
> or "don't take a ref". Surely that's not too onerous to add!?
>
> Actually the bigger part of it is actually the no-lazy mmu mode which
> is not yet used, I thought it was a neat little demonstrator of how code
> works with/without lazy but I will get rid of that for submission.

I admit that does add a bit more churn than necessary maybe that was
the main objection.

Here is the entire kernel/sched/core.c change after that is removed.
Pretty simple now. I'll resubmit.

Thanks,
Nick

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e359c76ea2e2..1be0b97e12ec 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4171,7 +4171,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
__releases(rq->lock)
{
struct rq *rq = this_rq();
- struct mm_struct *mm = rq->prev_mm;
+ struct mm_struct *mm = NULL;
long prev_state;

/*
@@ -4190,7 +4190,10 @@ static struct rq *finish_task_switch(struct task_struct *prev)
current->comm, current->pid, preempt_count()))
preempt_count_set(FORK_PREEMPT_COUNT);

- rq->prev_mm = NULL;
+#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT
+ mm = rq->prev_lazy_mm;
+ rq->prev_lazy_mm = NULL;
+#endif

/*
* A task struct has one reference for the use as "current".
@@ -4326,9 +4329,21 @@ context_switch(struct rq *rq, struct task_struct *prev,
switch_mm_irqs_off(prev->active_mm, next->mm, next);

if (!prev->mm) { // from kernel
- /* will mmdrop_lazy_tlb() in finish_task_switch(). */
- rq->prev_mm = prev->active_mm;
+#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT
+ /* Will mmdrop_lazy_tlb() in finish_task_switch(). */
+ rq->prev_lazy_mm = prev->active_mm;
prev->active_mm = NULL;
+#else
+ /*
+ * Without MMU_LAZY_TLB_REFCOUNT there is no lazy
+ * tracking (because no rq->prev_lazy_mm) in
+ * finish_task_switch, so no mmdrop_lazy_tlb(),
+ * so no memory barrier for membarrier (see the
+ * membarrier comment in finish_task_switch()).
+ * Do it here.
+ */
+ smp_mb();
+#endif
}
}

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a189bec13729..0729cf19a987 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -961,7 +961,9 @@ struct rq {
struct task_struct *idle;
struct task_struct *stop;
unsigned long next_balance;
- struct mm_struct *prev_mm;
+#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT
+ struct mm_struct *prev_lazy_mm;
+#endif

unsigned int clock_update_flags;
u64 clock;