Re: [RFC PATCH v1] mm: improve call_controls_lock

[JaeJoon Jung <rgbi3307@gmail.com>]

On Tue, 30 Dec 2025 at 00:23, SeongJae Park <sj@kernel.org> wrote:
>
> Hello Asier,
>
>
> Thank you for sending this patch!
>
> On Mon, 29 Dec 2025 14:55:32 +0000 Asier Gutierrez <gutierrez.asier@huawei-partners.com> wrote:
>
> > This is a minor patch set for a call_controls_lock synchronization improvement.
>
> Please break description lines to not exceed 75 characters per line.
>
> >
> > Spinlocks are faster than mutexes, even when the mutex takes the fast
> > path. Hence, this patch replaces the mutex call_controls_lock with a spinlock.
>
> But call_controls_lock is not being used on performance critical part.
> Actually, most of DAMON code is not performance critical.  I really appreciate
> your patch, but I have to say I don't think this change is really needed now.
> Please let me know if I'm missing something.

Paradoxically, when it comes to locking, spin_lock is better than
mutex_lock
because "most of DAMON code is not performance critical."

DAMON code only accesses the ctx belonging to kdamond itself. For
example:
kdamond.0 --> ctx.0
kdamond.1 --> ctx.1
kdamond.2 --> ctx.2
kdamond.# --> ctx.#

There is no cross-approach as shown below:
kdamond.0 --> ctx.1
kdamond.1 --> ctx.2
kdamond.2 --> ctx.0

Only the data belonging to kdamond needs to be resolved for concurrent access.
most DAMON code needs to lock/unlock briefly when add/del linked
lists,
so spin_lock is effective.  If you handle it with a mutex, it becomes
more
complicated because the rescheduling occurs as a context switch occurs
inside the kernel.  Moreover, since the call_controls_lock that is
currently
being raised as a problem only occurs in two places, the kdamon_call()
loop
and the damon_call() function, it is effective to handle it with a
spin_lock
as shown below.

@@ -1502,14 +1501,15 @@ int damon_call(struct damon_ctx *ctx, struct
damon_call_control *control)
        control->canceled = false;
        INIT_LIST_HEAD(&control->list);

-       mutex_lock(&ctx->call_controls_lock);
+       spin_lock(&ctx->call_controls_lock);
+       /* damon_is_running */
        if (ctx->kdamond) {
                list_add_tail(&control->list, &ctx->call_controls);
        } else {
-               mutex_unlock(&ctx->call_controls_lock);
+               spin_unlock(&ctx->call_controls_lock);
                return -EINVAL;
        }
-       mutex_unlock(&ctx->call_controls_lock);
+       spin_unlock(&ctx->call_controls_lock);

        if (control->repeat)
                return 0;

>
> >
> > Initial benchmarking shows the following results
> >
> >
> > # bpftrace -e 'kprobe:kdamond_call { @start[tid] = nsecs; }
>
> Commit log shouldn't start with '#'.  Please consider indenting the above
> command and below outputs of it.
>
>
> Thanks,
> SJ
>
> [...]
>