linux-kernel - Re: [RFC PATCH v2 1/3] meminfo_extra: introduce meminfo extra

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Date:   Sun, 29 Mar 2020 10:19:07 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Jaewon Kim <jaewon31.kim@...sung.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     vbabka@...e.cz, adobriyan@...il.com, akpm@...ux-foundation.org,
        labbott@...hat.com, sumit.semwal@...aro.org, minchan@...nel.org,
        ngupta@...are.org, sergey.senozhatsky.work@...il.com,
        kasong@...hat.com, bhe@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jaewon31.kim@...il.com,
        linux-api@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [RFC PATCH v2 1/3] meminfo_extra: introduce meminfo extra

On Tue, Mar 24, 2020 at 09:53:16PM +0900, Jaewon Kim wrote:
>
>
> On 2020년 03월 24일 20:46, Greg KH wrote:
> > On Tue, Mar 24, 2020 at 08:37:38PM +0900, Jaewon Kim wrote:
> >>
> >> On 2020년 03월 24일 19:11, Greg KH wrote:
> >>> On Tue, Mar 24, 2020 at 06:11:17PM +0900, Jaewon Kim wrote:
> >>>> On 2020년 03월 23일 18:53, Greg KH wrote:
> >>>>>> +int register_meminfo_extra(atomic_long_t *val, int shift, const char *name)
> >>>>>> +{
> >>>>>> +	struct meminfo_extra *meminfo, *memtemp;
> >>>>>> +	int len;
> >>>>>> +	int error = 0;
> >>>>>> +
> >>>>>> +	meminfo = kzalloc(sizeof(*meminfo), GFP_KERNEL);
> >>>>>> +	if (!meminfo) {
> >>>>>> +		error = -ENOMEM;
> >>>>>> +		goto out;
> >>>>>> +	}
> >>>>>> +
> >>>>>> +	meminfo->val = val;
> >>>>>> +	meminfo->shift_for_page = shift;
> >>>>>> +	strncpy(meminfo->name, name, NAME_SIZE);
> >>>>>> +	len = strlen(meminfo->name);
> >>>>>> +	meminfo->name[len] = ':';
> >>>>>> +	strncpy(meminfo->name_pad, meminfo->name, NAME_BUF_SIZE);
> >>>>>> +	while (++len < NAME_BUF_SIZE - 1)
> >>>>>> +		meminfo->name_pad[len] = ' ';
> >>>>>> +
> >>>>>> +	spin_lock(&meminfo_lock);
> >>>>>> +	list_for_each_entry_rcu(memtemp, &meminfo_head, list) {
> >>>>>> +		if (memtemp->val == val) {
> >>>>>> +			error = -EINVAL;
> >>>>>> +			break;
> >>>>>> +		}
> >>>>>> +	}
> >>>>>> +	if (!error)
> >>>>>> +		list_add_tail_rcu(&meminfo->list, &meminfo_head);
> >>>>>> +	spin_unlock(&meminfo_lock);
> >>>>> If you have a lock, why are you needing rcu?
> >>>> I think _rcu should be removed out of list_for_each_entry_rcu.
> >>>> But I'm confused about what you meant.
> >>>> I used rcu_read_lock on __meminfo_extra,
> >>>> and I think spin_lock is also needed for addition and deletion to handle multiple modifiers.
> >>> If that's the case, then that's fine, it just didn't seem like that was
> >>> needed.  Or I might have been reading your rcu logic incorrectly...
> >>>
> >>>>>> +	if (error)
> >>>>>> +		kfree(meminfo);
> >>>>>> +out:
> >>>>>> +
> >>>>>> +	return error;
> >>>>>> +}
> >>>>>> +EXPORT_SYMBOL(register_meminfo_extra);
> >>>>> EXPORT_SYMBOL_GPL()?  I have to ask :)
> >>>> I can use EXPORT_SYMBOL_GPL.
> >>>>> thanks,
> >>>>>
> >>>>> greg k-h
> >>>>>
> >>>>>
> >>>> Hello
> >>>> Thank you for your comment.
> >>>>
> >>>> By the way there was not resolved discussion on v1 patch as I mentioned on cover page.
> >>>> I'd like to hear your opinion on this /proc/meminfo_extra node.
> >>> I think it is the propagation of an old and obsolete interface that you
> >>> will have to support for the next 20+ years and yet not actually be
> >>> useful :)
> >>>
> >>>> Do you think this is meaningful or cannot co-exist with other future
> >>>> sysfs based API.
> >>> What sysfs-based API?
> >> Please refer to mail thread on v1 patch set - https://protect2.fireeye.com/url?k=16e3accc-4b2f6548-16e22783-0cc47aa8f5ba-935fe828ac2f6656&u=https://lkml.org/lkml/fancy/2020/3/10/2102
> >> especially discussion with Leon Romanovsky on https://protect2.fireeye.com/url?k=74208ed9-29ec475d-74210596-0cc47aa8f5ba-0bd4ef48931fec95&u=https://lkml.org/lkml/fancy/2020/3/16/140
> > I really do not understand what you are referring to here, sorry.   I do
> > not see any sysfs-based code in that thread.
> Sorry. I also did not see actual code.
> Hello Leon Romanovsky, could you elaborate your plan regarding sysfs stuff?

Sorry for being late, I wasn't in "TO:", so missed the whole discussion.

Greg,

We need the exposed information for the memory optimizations (debug, not
production) of our high speed NICs. Our devices (mlx5) allocates a lot of
memory, so optimization there can help us to scale in SRIOV mode easier and
be less constraint by the memory.

I want to emphasize that I don't like idea of extending /proc/* interface
because it is going to be painful to grep on large machines with many
devices. And I don't like the idea that every driver will need to register
into this interface, because it will be abused almost immediately.

My proposal was to create new sysfs file by driver/core and put all
information automatically there, for example, it can be
/sys/devices/pci0000:00/0000:00:0c.0/meminfo
                                     ^^^^^^^

Thanks