Re: [PATCH -mm v8 4/7] proc: add kpagecgroup file

On Thu, Jul 16, 2015 at 12:04:59PM -0700, Andres Lagar-Cavilla wrote:

On Thu, Jul 16, 2015 at 2:28 AM, Vladimir Davydov [off-list ref]
wrote:

quoted

On Wed, Jul 15, 2015 at 12:03:18PM -0700, Andres Lagar-Cavilla wrote:

quoted

For both /proc/kpage* interfaces you add (and more critically for the
rmap-causing one, kpageidle):

It's a good idea to do cond_sched(). Whether after each pfn, each Nth
pfn, each put_user, I leave to you, but a reasonable cadence is
needed, because user-space can call this on the entire physical
address space, and that's a lot of work to do without re-scheduling.

I really don't think it's necessary. These files can only be
read/written by the root, who has plenty ways to kill the system anyway.
The program that is allowed to read/write these files must be conscious
and do it in batches of reasonable size. AFAICS the same reasoning
already lays behind /proc/kpagecount and /proc/kpageflag, which also do
not thrust the "right" batch size on their readers.

Beg to disagree. You're conflating intended use with system health. A
cond_sched() is a one-liner.

I would still prefer not to clutter the code with cond_resched's, but I
don't think it's a matter worth arguing upon, so I'll prepare a patch
that makes all /proc/kapge* files issue cond_resched periodically and
leave it up to Andrew to decide if it should be applied or not.

Thanks,
Vladimir