linux-kernel - Re: [tip:timers/core] [posix] 1535cb8028: stress-ng.epoll.ops_per

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Message-ID: <20250327204512.548d2507@pumpkin>
Date: Thu, 27 Mar 2025 20:45:12 +0000
From: David Laight <david.laight.linux@...il.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, kernel test robot <oliver.sang@...el.com>,
 oe-lkp@...ts.linux.dev, lkp@...el.com, linux-kernel@...r.kernel.org,
 x86@...nel.org, Benjamin Segall <bsegall@...gle.com>, Frederic Weisbecker
 <frederic@...nel.org>
Subject: Re: [tip:timers/core] [posix] 1535cb8028:
 stress-ng.epoll.ops_per_sec 36.2% regression

On Thu, 27 Mar 2025 14:48:37 +0100
Mateusz Guzik <mjguzik@...il.com> wrote:

> On Thu, Mar 27, 2025 at 2:44 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Thu, Mar 27, 2025 at 2:43 PM Mateusz Guzik <mjguzik@...il.com> wrote:  
> > >
> > > On Thu, Mar 27, 2025 at 2:17 PM Eric Dumazet <edumazet@...gle.com> wrote:  
> > > >
> > > > On Thu, Mar 27, 2025 at 2:14 PM Thomas Gleixner <tglx@...utronix.de> wrote:  
> > > > >
> > > > > On Thu, Mar 27 2025 at 12:37, Eric Dumazet wrote:  
> > > > > > On Thu, Mar 27, 2025 at 11:50 AM Thomas Gleixner <tglx@...utronix.de> wrote:  
> > > > > >> Cute. How much bloat does it cause?  
> > > > > >
> > > > > > This would expand 'struct ucounts' by 192 bytes on x86, if the patch
> > > > > > was actually working :)
> > > > > >
> > > > > > Note sure if it is feasible without something more intrusive like  
> > > > >
> > > > > I'm not sure about the actual benefit. The problem is that parallel
> > > > > invocations which access the same ucount still will run into contention
> > > > > of the cache line they are modifying.
> > > > >
> > > > > For the signal case, all invocations increment rlimit[SIGPENDING], so
> > > > > putting that into a different cache line does not buy a lot.
> > > > >
> > > > > False sharing is when you have a lot of hot path readers on some other
> > > > > member of the data structure, which happens to share the cache line with
> > > > > the modified member. But that's not really the case here.  
> > > >
> > > > We applications stressing all the counters at the same time (from
> > > > different threads)
> > > >
> > > > You seem to focus on posix timers only :)  
> > >
> > > Well in that case:
> > > (gdb) ptype /o struct ucounts
> > > /* offset      |    size */  type = struct ucounts {
> > > /*      0      |      16 */    struct hlist_node {
> > > /*      0      |       8 */        struct hlist_node *next;
> > > /*      8      |       8 */        struct hlist_node **pprev;
> > >
> > >                                    /* total size (bytes):   16 */
> > >                                } node;
> > > /*     16      |       8 */    struct user_namespace *ns;
> > > /*     24      |       4 */    kuid_t uid;
> > > /*     28      |       4 */    atomic_t count;
> > > /*     32      |      96 */    atomic_long_t ucount[12];
> > > /*    128      |     256 */    struct {
> > > /*      0      |       8 */        atomic_long_t val;
> > >                                } rlimit[4];
> > >
> > >                                /* total size (bytes):  384 */
> > >                              }
> > >
> > > This comes from malloc. Given 384 bytes of size it is going to be
> > > backed by a 512-byte sized buffer -- that's a clear cut waste of 128
> > > bytes.
> > >
> > > It is plausible creating a 384-byte sized slab for kmalloc would help
> > > save memory overall (not just for this specific struct), but that
> > > would require extensive testing in real workloads. I think Google is
> > > in position to do it on their fleet and android? fwiw Solaris and
> > > FreeBSD do have slabs of this size and it does save memory over there.
> > > I understand it is a tradeoff, hence I'm not claiming this needs to be
> > > added. I do claim it does warrant evaluation, but I wont blame anyone
> > > for not wanting to do dig into it.
> > >
> > > The other option is to lean into it. In this case I point out the
> > > refcount shares the cacheline with some of the limits and that it
> > > could be moved to a dedicated line while still keeping the struct <
> > > 512 bytes, thus not spending more memory on allocation. the refcount
> > > changes less frequently than limits themselves so it's not a big deal,
> > > but it can be adjusted "for free" if you will.
> > >
> > > while here I would probably change the name of the field. A reference
> > > counter named "count" in a struct named "ucounts", followed by an
> > > "ucount" array is rather unpleasing. How about s/count/refcount?  
> >
> >
> > How many 'struct ucounts' are in use in a typical host ?
> >
> > Compared to other costs, this seems pure noise to me.  
> 
> I did not claim this is going to increase memory usage in a significant manner.
> 
> I claim regardless of this change a 384-byte slab for kmalloc may be
> saving memory and this bit may be enough of an excuse to evaluate it,
> should someone be interested.
> 
> Apart from that I claim that if the 512-byte is going to be used to
> back the 384 bytes used by the struct, the patch can trivially move
> the refcount to a dedicated cacheline to avoid some of the bouncing
> and still fit in the 512-byte allocation. I see no reason to not do
> it.
> 

What about systems with much larger cache lines?

	David