linux-kernel - Re: [PATCH v2 00/12] uprobes: add batched register/unregister APIs and per-CPU RW semaphore

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Message-ID: <4304cf1f-26e8-44c5-9755-bc7c526dcd7b@paulmck-laptop>
Date: Wed, 3 Jul 2024 15:07:01 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	linux-trace-kernel@...r.kernel.org, mhiramat@...nel.org,
	oleg@...hat.com, mingo@...hat.com, bpf@...r.kernel.org,
	jolsa@...nel.org, clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 00/12] uprobes: add batched register/unregister APIs
 and per-CPU RW semaphore

On Wed, Jul 03, 2024 at 05:57:54PM -0400, Steven Rostedt wrote:
> On Wed, 3 Jul 2024 09:50:57 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > However, in the past, the memory-barrier and array-indexing overhead
> > > of SRCU has made it a no-go for lightweight probes into fastpath code.
> > > And these cases were what motivated RCU Tasks Trace (as opposed to RCU
> > > Tasks Rude).  
> > 
> > I'm thinking we're growing too many RCU flavours again :/ I suppose I'll
> > have to go read up on rcu/tasks.* and see what's what.
> 
> This RCU flavor is the one to handle trampolines. If the trampoline
> never voluntarily schedules, then the quiescent state is a voluntary
> schedule. The issue with trampolines is that if something was preempted
> as it was jumping to a trampoline, there's no way to know when it is
> safe to free that trampoline, as some preempted task's next instruction
> is on that trampoline.
> 
> Any trampoline that does not voluntary schedule can use RCU task
> synchronization. As it will wait till all tasks have voluntarily
> scheduled or have entered user space (IIRC, Paul can correct me if I'm
> wrong).

Agreed!

> Now, if a trampoline does schedule, it would need to incorporate some
> ref counting on the trampoline to handle the scheduling, but could
> still use RCU task synchronization up to the point of the ref count.

Or, if the schedule is due at most to a page fault, it can use RCU
Tasks Trace.

> And yes, the rude flavor was to handle the !rcu_is_watching case, and
> can now be removed.

>From x86, agreed.

But have the other architectures done all the inlining and addition of
noistr required to permit this?  (Maybe they have, I honestly do not know.
But last I checked a few months ago, ARMv8 was not ready yet.)

							Thanx, Paul