live-patching.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: gor@linux.ibm.com, jpoimboe@redhat.com, jikos@kernel.org,
	mbenes@suse.cz, pmladek@suse.com, mingo@kernel.org,
	linux-kernel@vger.kernel.org, joe.lawrence@redhat.com,
	fweisbec@gmail.com, tglx@linutronix.de, hca@linux.ibm.com,
	svens@linux.ibm.com, sumanthk@linux.ibm.com,
	live-patching@vger.kernel.org
Subject: Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU
Date: Wed, 22 Sep 2021 12:59:38 -0700	[thread overview]
Message-ID: <20210922195938.GD880162@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <YUuIb12XCQlBfIQW@hirez.programming.kicks-ass.net>

On Wed, Sep 22, 2021 at 09:47:59PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 22, 2021 at 09:33:43PM +0200, Peter Zijlstra wrote:
> 
> > Anyway, lemme see if I get your proposal; lets say the counter starts at
> > 0 and is in kernel space.
> > 
> >  0x00(0) - kernel
> >  0x02(2) - user
> >  0x04(0) - kernel
> > 
> > So far so simple, then NMI on top of that goes:
> > 
> >  0x00(0) - kernel
> >  0x03(3) - kernel + nmi
> >  0x04(0) - kernel
> >  0x06(2) - user
> >  0x09(1) - user + nmi
> >  0x0a(2) - user
> > 
> > Which then gives us:
> > 
> >  (0) := kernel
> >  (1) := nmi-from-user
> >  (2) := user
> >  (3) := nmi-from-kernel
> > 
> > Which should work I suppose. But like I said above, I'd be happier if
> > this counter would live in context_tracking rather than RCU.
> 
> Furthermore, if we have this counter, the we can also do things like:
> 
>   seq = context_tracking_seq_cpu(that_cpu);
>   if ((seq & 3) != USER)
>     // nohz_fail, do something
>   set_tsk_thread_flag(curr_task(that_cpu), TIF_DO_SOME_WORK);
>   if (seq == context_tracking_seq_cpu(that_cpu))
>     // success!!
> 
> To remotely set pending state. Allowing yet more NOHZ_FULL fixes, like,
> for example, eliding the text_poke IPIs.

Nice!

There have been several instances where I thought that the extra state
would help RCU, but each time there turned out to be a simpler way to
get things done.  Or that it eventually turned out that RCU didn't need
to care about the difference between idle and nohz_full userspace.

							Thanx, Paul

  reply	other threads:[~2021-09-22 19:59 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-22 11:05 [RFC][PATCH 0/7] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL cpus Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 1/7] sched,rcu: Rework try_invoke_on_locked_down_task() Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 2/7] sched: Fix task_try_func() Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 3/7] sched,livepatch: Use task_try_func() Peter Zijlstra
2021-09-23 12:05   ` Petr Mladek
2021-09-23 13:17     ` Peter Zijlstra
2021-09-23 13:47       ` Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 4/7] sched: Simplify wake_up_*idle*() Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 5/7] sched,livepatch: Use wake_up_if_idle() Peter Zijlstra
2021-09-22 13:05   ` Miroslav Benes
2021-09-23 12:19     ` Petr Mladek
2021-09-22 11:05 ` [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU Peter Zijlstra
2021-09-22 15:17   ` Paul E. McKenney
2021-09-22 19:33     ` Peter Zijlstra
2021-09-22 19:47       ` Peter Zijlstra
2021-09-22 19:59         ` Paul E. McKenney [this message]
2021-09-22 19:53       ` Paul E. McKenney
2021-09-22 20:02         ` Peter Zijlstra
2021-09-23 12:10   ` Petr Mladek
2021-09-24 18:57   ` Joel Savitz
2021-09-27 14:33     ` Petr Mladek
2021-09-22 11:05 ` [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks Peter Zijlstra
2021-09-23 13:14   ` Petr Mladek
2021-09-23 13:28     ` Peter Zijlstra
2021-09-24  7:33       ` Petr Mladek
2021-09-23 13:22 ` [RFC][PATCH 0/7] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL cpus Petr Mladek

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210922195938.GD880162@paulmck-ThinkPad-P17-Gen-1 \
    --to=paulmck@kernel.org \
    --cc=fweisbec@gmail.com \
    --cc=gor@linux.ibm.com \
    --cc=hca@linux.ibm.com \
    --cc=jikos@kernel.org \
    --cc=joe.lawrence@redhat.com \
    --cc=jpoimboe@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=live-patching@vger.kernel.org \
    --cc=mbenes@suse.cz \
    --cc=mingo@kernel.org \
    --cc=peterz@infradead.org \
    --cc=pmladek@suse.com \
    --cc=sumanthk@linux.ibm.com \
    --cc=svens@linux.ibm.com \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).