linux-kernel - Re: [PATCH v2] locking/local_lock, mm: Replace localtry_ helpers with local_trylock

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Message-ID: <CAADnVQ+m12aU3qWJMTECOTa=B7A_UFSLk4v8MAcr4ZaN5EHdNg@mail.gmail.com>
Date: Thu, 3 Apr 2025 07:45:12 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	bpf <bpf@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Sebastian Sewior <bigeasy@...utronix.de>, Steven Rostedt <rostedt@...dmis.org>, 
	Michal Hocko <mhocko@...e.com>, linux-mm <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] locking/local_lock, mm: Replace localtry_ helpers with
 local_trylock_t type

On Thu, Apr 3, 2025 at 2:26 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 4/2/25 23:40, Alexei Starovoitov wrote:
> > On Wed, Apr 2, 2025 at 1:56 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >>
> >> On Tue, Apr 01, 2025 at 01:52:45PM -0700, Alexei Starovoitov wrote:
> >> > From: Alexei Starovoitov <ast@...nel.org>
> >> >
> >> > Partially revert commit 0aaddfb06882 ("locking/local_lock: Introduce localtry_lock_t").
> >> > Remove localtry_*() helpers, since localtry_lock() name might
> >> > be misinterpreted as "try lock".
> >> >
> >> > Introduce local_trylock[_irqsave]() helpers that only work
> >> > with newly introduced local_trylock_t type.
> >> > Note that attempt to use local_trylock[_irqsave]() with local_lock_t
> >> > will cause compilation failure.
> >> >
> >> > Usage and behavior in !PREEMPT_RT:
> >> >
> >> > local_lock_t lock;                     // sizeof(lock) == 0
> >> > local_lock(&lock);                     // preempt disable
> >> > local_lock_irqsave(&lock, ...);        // irq save
> >> > if (local_trylock_irqsave(&lock, ...)) // compilation error
> >> >
> >> > local_trylock_t lock;                  // sizeof(lock) == 4
> >>
> >> Is there a reason for this 'acquired' to be int? Can it be uint8_t? No
> >> need to change anything here but I plan to change it later to compact as
> >> much as possible within one (or two) cachline for memcg stocks.
> >
> > I don't see any issue. I can make it u8 right away.
>
> Are you planning to put the lock near other <64bit sized values in memcg
> stock? Otherwise it will be padded anyway?
>
> I hope it won't hurt the performance though, AFAIK at least sub-word atomics
> are much slower than using a full word. But we use only read/write once for
> acquired so hopefully it's fine?

Sub-words atomics are slow, but these are not atomics.
Just plain load/store of u8. Should be fine.