Re: SD_LOAD_BALANCE

[Julia Lawall]

On Thu, 3 Sep 2020, Valentin Schneider wrote:

>
> Hi Julia,
>
> On 03/09/20 15:09, Julia Lawall wrote:
> > Uses of SD_LOAD_BALANCE were removed in commit e669ac8ab952 (first
> > released in v5.8), with the comment:
> >
> > The SD_LOAD_BALANCE flag is set unconditionally for all domains in
> > sd_init().
> >
> > I have the impression that this was not quite true.  The NUMA domain was
> > not initialized with sd_init, and didn't have the SD_LOAD_BALANCE flag
> > set.
>
> Did you check the contents of
>
>   /proc/sys/kernel/sched_domain/cpu*/domain*/flags
>
> (requires CONFIG_SCHED_DEBUG)? If the LSB is set, it would mean
> SD_LOAD_BALANCE is set.
>
> The sched_domain construction flow isn't the easiest thing to follow, but
> NUMA domains *have* to go through sd_init().
>
> What happens is we first go through sched_init_numa(), and there we add
> some more topology levels on top of the default ones (or the arch-defined
> ones if using an arch-defined topology hierarchy) by using the NUMA
> distance table.
>
> We then build the actual domains in sched_init_domains(), and that goes
> through a loop that looks like
>
>   for_each_cpu() {
>       for_each_sd_topology() {
>           build_sched_domain() -> sd_init()
>       }
>   }
>
> where the SD topology loop is going to iterate over the newly-added
> NUMA-specific topology levels. Since that used to unconditionally set
> SD_LOAD_BALANCE, NUMA domains really ought to have it.
>
> If that wasn't the case, we would have fired the (now removed) warning in
> sched_domain_debug_one() that would do:
>
>        if (!(sd->flags & SD_LOAD_BALANCE)) {
>                printk("does not load-balance\n");
>                if (sd->parent)
>                        printk(KERN_ERR "ERROR: !SD_LOAD_BALANCE domain has parent");
>                return -1;
>        }

OK, I see also that in v5.7 tmp->flags does have the SD_LOAD_BALANCE bit
set, so I will have to look further to see what the difference is.  Thanks
for the pointers.

julia


>
> > The effect is that in v5.8, the for_each_domain loop in
> > select_task_rq_fair can always end up at the global NUMA domain, and thus
> > consider any pair of waking cpu (cpu) and previous cpus of the wakee
> > (prev_cpu) as arguments to wake_affine.  Up to v5.7, this was only
> > possible if cpu and prev_cpu were together in some lower level domain, ie
> > sharing the LLC.  The effect is that in v5.8 wake_affine can often end up
> > choosing as a target a core that does not share the LLC with the core
> > where the thread ran previously.  Threads then move around a lot between
> > the different sockets.
> >
> > Was this intentional?
> >
>
> AFAICT it isn't forbidden for the logic here to peek outside of the
> previous LLC. The NUMA reclaim distance thing says we allow affine wakeups
> and fork / exec balancing to move a task to a CPU at most RECLAIM_DISTANCE
> away (in NUMA distance values). However, I don't remember any patch
> changing this between v5.7 and v5.8.
>
> Briefly glancing over the kernel/sched log between v5.7 and v5.8, I don't
> see any obvious culprits. Did you try to bisect this? If it indeed ends on
> the SD_LOAD_BALANCE thing, well, I'll be off eating my keyboard.
>
> > The effect can be seen in the traces of the parsec vips benchmark at the
> > following URL:
> >
> > https://pages.lip6.fr/Julia.Lawall/vips.pdf
> >
> > The first two graphs (complete run and small fragment) are Linux v5.7 and
> > the next two are Linux v5.8.  The machine has 160 hardware threads
> > organized in 4 sockets and the colors are by socket.  In the small
> > fragment for v5.7 (second graph), one can see that a given pid pretty much
> > stays on the same socket, while in the corresponding fragment for v5.8
> > (fourth graph), the pids move around between the sockets.  The x's
> > describe the unblocks that result in a migration.  A pink x means that the
> > migration is in the same socket, while a blue x means that the migration
> > is to another socket. It's not apparent from the graphs, but by adding
> > some tracing, it seems that the new socket is always the one of the core
> > that handles the wakeup.
> >
>
> Interesting graphs, thanks for sharing!
>
> > I haven't yet studied the early part of the execution of vips in detail,
> > but I suspect that the same issue causes all of the threads to be
> > initially on the same socket in v5.7, while in v5.8 they are more quickly
> > dispersed to other sockets.
> >
> > My impression from the parsec and the NAS benchmarks is that the v5.8
> > performance is a bit better than v5.7, probably because of getting more
> > threads to more different sockets earlier, but other benchmarks might
> > rely more on locality and might react less well to threads moving around
> > so much in this way.
> >
> > julia
>