linux-kernel - Re: [PATCH] ftrace: As disabling interrupts is costly and write_lock variant of tasklist_lock is not held from interrupt context it is not necessary to disable interrupts.

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Message-ID: <CACkrGNWtkpcuBMED6etCG8K7O3QkDFHz-rT2GQ8GGG65CH0gcw@mail.gmail.com>
Date:	Thu, 19 May 2016 17:10:44 +0530
From:	Maloy Ghosh <maloy021189ju@...il.com>
To:	Soumya PN <soumya.p.n@....com>
Cc:	rostedt@...dmis.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, chandru.muthu@....com
Subject: Re: [PATCH] ftrace: As disabling interrupts is costly and write_lock
 variant of tasklist_lock is not held from interrupt context it is not
 necessary to disable interrupts.

Hi,
I guess (from the error code) this is to do with your patch not having
short one line subject before the detailed explanation of what your
patch does. The automated script might be treating your patch detailed
description as subject line. More can be found in the patch submitting
guideline.

Regards,

On Thu, May 19, 2016 at 6:10 PM, Soumya PN <soumya.p.n@....com> wrote:
> In ftrace.c inside the function alloc_retstack_tasklist()(which will be
> invoked when function_graph tracing is on) the tasklist_lock is being
> held as reader while iterating through list of threads. Here the lock
> being held as reader with irqs disabled. The tasklist_lock is never
> write_locked in interrupt context so it is safe to not disable interrupts
> for the duration of read_lock in this block which, can be significant,
> given the block of code iterates through all threads. Hence changing the
> code to call read_lock() and read_unlock() instead of read_lock_irqsave()
> and read_unlock_irqrestore(). Seen the same change is made in
> "kernel/tracepoint.c" and "kernel/sched/core.c"files with commit ids
> 'commit 8063e41d2ffc ("tracing: Change syscall_*regfunc() to check
> PF_KTHREAD and use for_each_process_thread()")'
> 'commit 3472eaa1f12e ("sched: normalize_rt_tasks(): Don't use _irqsave
> for tasklist_lock, use task_rq_lock()")'
>
> Done basic testing by enabling function graph tracing in x86_64.
>
> Signed-off-by: Soumya PN <soumya.p.n@....com>
> ---
>  kernel/trace/ftrace.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index b1870fb..45bc72f 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5713,7 +5713,6 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
>  {
>         int i;
>         int ret = 0;
> -       unsigned long flags;
>         int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
>         struct task_struct *g, *t;
>
> @@ -5728,8 +5727,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
>                         goto free;
>                 }
>         }
> -
> -       read_lock_irqsave(&tasklist_lock, flags);
> +       read_lock(&tasklist_lock);
>         do_each_thread(g, t) {
>                 if (start == end) {
>                         ret = -EAGAIN;
> @@ -5747,7 +5745,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
>         } while_each_thread(g, t);
>
>  unlock:
> -       read_unlock_irqrestore(&tasklist_lock, flags);
> +       read_unlock(&tasklist_lock);
>  free:
>         for (i = start; i < end; i++)
>                 kfree(ret_stack_list[i]);
> --
> 1.8.3.1
>



-- 
--
Maloy Ghosh
Research Engineer
Next Generation Network (NGN)
Center for Development of Telematics (CDoT)