linux-kernel - Re: [PATCH 21/21] binder: switch alloc->mutex to spinlock

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Message-ID: <20231107090849.262070-1-aliceryhl@google.com>
Date:   Tue,  7 Nov 2023 09:08:49 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     Carlos Llamas <cmllamas@...gle.com>
Cc:     "Arve Hjønnevåg" <arve@...roid.com>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        Martijn Coenen <maco@...roid.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Todd Kjos <tkjos@...roid.com>
Subject: Re: [PATCH 21/21] binder: switch alloc->mutex to spinlock_t

Carlos Llamas <cmllamas@...gle.com> writes:
> The alloc->mutex is a highly contended lock that causes performance
> issues on Android devices. When a low-priority task is given this lock
> and it sleeps, it becomes difficult for the task to wakeup and complete
> its work. This delays other tasks that are also waiting on the mutex.

Grammar nit: "to wake up"

> The problem gets worse when there is memory pressure in the system,
> because this increases the contention on the alloc->mutex while the
> shrinker reclaims binder pages.
> 
> Switching to a spinlock helps to keep the waiters running and avoids the
> overhead of waking up tasks. This significantly improves the transaction
> latency when the problematic scenario occurs.
>
> [snip]
>
> Note that it is only possible to convert this lock after a series of
> changes made by previous patches. These mainly include refactoring the
> sections that might_sleep() and changing the locking order with the
> mmap_lock amongst others.
> 
> Signed-off-by: Carlos Llamas <cmllamas@...gle.com>

Nice!

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

>  	 * binder_free_buf_locked(). However, that could
> -	 * increase contention for the alloc mutex if clear_on_free
> -	 * is used frequently for large buffers. The mutex is not
> +	 * increase contention for the alloc->lock if clear_on_free
> +	 * is used frequently for large buffers. This lock is not

Grammar nit: Shouldn't this say "However, that could increase contention
on alloc->lock if clear_on_free is used frequently for large buffers."?

Alice