linux-kernel - Re: [PATCH v2 3/7] mm/swap: Use local

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Date:   Mon, 25 May 2020 08:44:36 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH v2 3/7] mm/swap: Use local_lock for protection



* Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> From: Ingo Molnar <mingo@...nel.org>
> 
> The various struct pagevec per CPU variables are protected by disabling
> either preemption or interrupts across the critical sections. Inside
> these sections spinlocks have to be acquired.

> diff --git a/mm/swap.c b/mm/swap.c
> index bf9a79fed62d7..4f965292044ca 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -35,6 +35,7 @@
>  #include <linux/uio.h>
>  #include <linux/hugetlb.h>
>  #include <linux/page_idle.h>
> +#include <linux/locallock.h>
>  
>  #include "internal.h"
>  
> @@ -44,14 +45,29 @@
>  /* How many pages do we try to swap or page in/out together? */
>  int page_cluster;
>  
> -static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
> -static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
> -static DEFINE_PER_CPU(struct pagevec, lru_deactivate_file_pvecs);
> -static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);
> -static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);
> +/* Protecting lru_rotate_pvecs */
> +struct lru_rotate_pvecs {
> +	struct local_lock lock;
> +	struct pagevec pvec;
> +};
> +static DEFINE_PER_CPU(struct lru_rotate_pvecs, lru_rotate_pvecs) = {
> +	.lock = INIT_LOCAL_LOCK(lock),
> +};
> +
> +/* Protecting the following struct pagevec */
> +struct lru_pvecs {
> +	struct local_lock lock;
> +	struct pagevec lru_add_pvec;
> +	struct pagevec lru_deactivate_file_pvecs;
> +	struct pagevec lru_deactivate_pvecs;
> +	struct pagevec lru_lazyfree_pvecs;

Ack on coalescing these into the 'struct lru_pvecs' helper structure, 
but a minor namespace organization nit: I'd drop the _pvec/_pvecs 
postfix from the field names, i.e. make it something like this:

/* Protecting the following struct pagevec */
struct lru_pvecs {
	struct local_lock lock;
	struct pagevec lru_add;
	struct pagevec lru_deactivate_file;
	struct pagevec lru_deactivate;
	struct pagevec lru_lazyfree;

With that change, usage is a straightforward:

   pvec->lru_deactivate

instead of the double-pvec name:

   pvec->lru_deactivate_pvec

> +		local_lock_irqsave(&lru_rotate_pvecs.lock, flags);


Also:

> +		pvec = this_cpu_ptr(&lru_rotate_pvecs.pvec);

s/lru_rotate_pvecs
 /lru_rotate_pvec

it's a single pagevec, using plural is confusing when reading the 
code.

I'd also suggest adding a comment explaining why the lru_rotate_pvec 
local lock is split away from lru_add/deactivate/deactivate/lazyfree 
pagevec local lock.

Thanks,

	Ingo