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

[Andy Lutomirski]

> On Dec 2, 2020, at 6:20 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Sun, Nov 29, 2020 at 02:01:39AM +1000, Nicholas Piggin wrote:
>> +         * - A delayed freeing and RCU-like quiescing sequence based on
>> +         *   mm switching to avoid IPIs completely.
> 
> That one's interesting too. so basically you want to count switch_mm()
> invocations on each CPU. Then, periodically snapshot the counter on each
> CPU, and when they've all changed, increment a global counter.
> 
> Then, you snapshot the global counter and wait for it to increment
> (twice I think, the first increment might already be in progress).
> 
> The only question here is what should drive this machinery.. the tick
> probably.
> 
> This shouldn't be too hard to do I think.
> 
> Something a little like so perhaps?

I don’t think this will work.  A CPU can go idle with lazy mm and nohz forever.  This could lead to unbounded memory use on a lightly loaded system.