Re: [PATCH v6 6/6] rwsem: do optimistic spinning for writer lockacquisition

[Ingo Molnar]

* Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:

> We want to add optimistic spinning to rwsems because
> the writer rwsem does not perform as well as mutexes. Tim noticed that
> for exim (mail server) workloads, when reverting commit 4fc3f1d6 and
> Davidlohr noticed it when converting the i_mmap_mutex to a rwsem in some
> aim7 workloads. We've noticed that the biggest difference
> is when we fail to acquire a mutex in the fastpath, optimistic spinning
> comes in to play and we can avoid a large amount of unnecessary sleeping
> and overhead of moving tasks in and out of wait queue.
> 
> Allowing optimistic spinning before putting the writer on the wait queue
> reduces wait queue contention and provided greater chance for the rwsem
> to get acquired. With these changes, rwsem is on par with mutex.
> 
> Reviewed-by: Ingo Molnar <mingo@xxxxxxx>
> Reviewed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Reviewed-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> ---
>  include/linux/rwsem.h |    6 +-
>  kernel/rwsem.c        |   19 +++++-
>  lib/rwsem.c           |  203 ++++++++++++++++++++++++++++++++++++++++++++-----
>  3 files changed, 207 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 0616ffe..ef5a83a 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -26,6 +26,8 @@ struct rw_semaphore {
>  	long			count;
>  	raw_spinlock_t		wait_lock;
>  	struct list_head	wait_list;
> +	struct task_struct	*owner; /* write owner */
> +	void			*spin_mlock;

> +#define MLOCK(rwsem)    ((struct mcs_spin_node **)&((rwsem)->spin_mlock))

> +		mcs_spin_lock(MLOCK(sem), &node);

> +			mcs_spin_unlock(MLOCK(sem), &node);

> +			mcs_spin_unlock(MLOCK(sem), &node);

> +		mcs_spin_unlock(MLOCK(sem), &node);

That forced type casting is ugly and fragile.

To avoid having to include mcslock.h into rwsem.h just add a forward 
struct declaration, before the struct rw_semaphore definition:

struct mcs_spin_node;

Then define spin_mlock with the right type:

	struct mcs_spin_node		*spin_mlock;

I'd also suggest renaming 'spin_mlock', to reduce unnecessary variants. If 
the lock type name is 'struct mcs_spin_node' then 'mcs_lock' would be a 
perfect field name, right?

While at it, renaming mcs_spin_node to mcs_spinlock might be wise as well, 
and the include file would be named mcs_spinlock.h.

Thanks,

	Ingo
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