linux-kernel - Re: [PATCH v2] btrfs: zstd: use spin

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Date:   Thu, 14 Apr 2022 00:03:05 +0800
From:   Schspa Shi <schspa@...il.com>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     dsterba@...e.cz, clm@...com, dsterba@...e.com,
        josef@...icpanda.com, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, terrelln@...com
Subject: Re: [PATCH v2] btrfs: zstd: use spin_lock in timer callback

Nikolay Borisov <nborisov@...e.com> writes:

> On 11.04.22 г. 18:55 ч., Schspa Shi wrote:
>> This is an optimization for fix fee13fe96529 ("btrfs:
>> correct zstd workspace manager lock to use spin_lock_bh()")
>> The critical region for wsm.lock is only accessed by the process context and
>> the softirq context.
>> Because in the soft interrupt, the critical section will not be preempted by
>> the
>> soft interrupt again, there is no need to call spin_lock_bh(&wsm.lock) to turn
>> off the soft interrupt, spin_lock(&wsm.lock) is enough for this situation.
>> Changelog:
>> v1 -> v2:
>>      - Change the commit message to make it more readable.
>> [1] https://lore.kernel.org/all/20220408181523.92322-1-schspa@gmail.com/
>> Signed-off-by: Schspa Shi <schspa@...il.com>
>
> Has there been any measurable impact by this change? While it's correct it does mean that
>  someone looking at the code would see that in one call site we use plain spinlock and in
> another a _bh version and this is somewhat inconsistent.
>
Yes, it may seem a little confused. but it's allowed to save some
little peace of CPU times.
and "static inline void red_adaptative_timer(struct timer_list *t) in
net/sched/sch_red.c"
have similar usage.

> What's more I believe this is a noop since when softirqs are executing preemptible() would
> be false due to preempt_count() being non-0 and in the bh-disabling code
> in the spinlock we have:
>
>  /* First entry of a task into a BH disabled section? */
>     1         if (!current->softirq_disable_cnt) {
>   167                 if (preemptible()) {
>     1                         local_lock(&softirq_ctrl.lock);
>     2                         /* Required to meet the RCU bottomhalf requirements. */
>     3                         rcu_read_lock();
>     4                 } else {
>     5                         DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));
>     6                 }
>     7         }
>
>
> In this case we'd hit the else branch.

We won't hit the else branch. because current->softirq_disable_cnt
won't be zero in the origin case.

__do_softirq(void)
        softirq_handle_begin(void)
        __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
                        current->softirq_disable_cnt will be > 0 at this time.
    ......
        zstd_reclaim_timer_fn(struct timer_list *timer)
                        spin_lock_bh(&wsm.lock);
                        __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
                        if (!current->softirq_disable_cnt) {
                                                // this if branch won't hit
                                        }

        softirq_handle_end();

In this case, the "__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);"
won't do anything useful it only
increase softirq disable depth and decrease it in
"__local_bh_enable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);".

So it's safe to replace spin_lock_bh with spin_lock in a timer
callback function.


For the ksoftirqd, it's all the same.