linux-kernel - Re: [PATCH] rcu/nocb: Add an option to ON/OFF an offloading from RT context

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20220511102957.56bd582b@gandalf.local.home>
Date:   Wed, 11 May 2022 10:29:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Alison Chaiken <achaiken@...ora.tech>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraj.iitr10@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH] rcu/nocb: Add an option to ON/OFF an offloading from RT
 context

On Wed, 11 May 2022 15:39:56 +0200
Uladzislau Rezki <urezki@...il.com> wrote:

> <snip>
>  rcuop/6-54  [000] .N..  183.753018: rcu_invoke_callback:  rcu_preempt rhp=0xffffff88ffd440b0 func=__d_free.cfi_jt
>  rcuop/6-54  [000] .N..  183.753020: rcu_invoke_callback:  rcu_preempt rhp=0xffffff892ffd8400 func=inode_free_by_rcu.cfi_jt
>  rcuop/6-54  [000] .N..  183.753021: rcu_invoke_callback:  rcu_preempt rhp=0xffffff89327cd708 func=i_callback.cfi_jt
>  ... 
>  rcuop/6-54  [000] .N..  183.755941: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c5a968 func=i_callback.cfi_jt
>  rcuop/6-54  [000] .N..  183.755942: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c4bd20 func=__d_free.cfi_jt
>  rcuop/6-54  [000] dN..  183.755944: rcu_batch_end:        rcu_preempt CBs-invoked=2112 idle=>c<>c<>c<>c<
>  rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      Start context switch
>  rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      End context switch
> <snip>
> 
> i spent some time in order to understand why the context was not switched,
> even though the "rcuop" kthread was marked as TIF_NEED_RESCHED and an IPI
> was sent to the CPU_0 to reschedule. The last "." in latency field shows
> that a context has not disabled any preemption. So everything should be fine.
> 
> An explanation is that a local_bh_disable() modifies the current_thread_info()->preempt.count
> so a task becomes non preemtable but the ftrace does not provide any signal about
> it. So i was fooled for some time by my tracer logs.
> 
> Do you have any thoughts about it? Should it be solved or signaled
> somehow that a task in fact is not preemtable if a counter > 0?

Hmm, it should show it in the first part (where the 'd' is). Is this a
snapshot from the kernel or from trace-cmd?

-- Steve