linux-kernel - Re: [RFC PATCH v3 1/7] ktask: add documentation

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Date:   Fri, 8 Dec 2017 08:46:33 -0500
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        aaron.lu@...el.com, akpm@...ux-foundation.org,
        dave.hansen@...ux.intel.com, mgorman@...hsingularity.net,
        mike.kravetz@...cle.com, pasha.tatashin@...cle.com,
        steven.sistare@...cle.com, tim.c.chen@...el.com
Subject: Re: [RFC PATCH v3 1/7] ktask: add documentation

On 12/08/2017 07:43 AM, Michal Hocko wrote:
> On Wed 06-12-17 15:32:48, Daniel Jordan wrote:
>> On 12/06/2017 09:35 AM, Michal Hocko wrote:
> [...]
>>> There is also no mention about other
>>> characteristics (e.g. power management), resource isloataion etc. So > let me ask again. How do you control that the parallelized operation
>>> doesn't run outside of the limit imposed to the calling context?
>>
>> The current code doesn't do this, and the answer is the same for the rest of
>> your questions.
> 
> I really believe this should be addressed before this can be considered
> for merging. While what you have might be sufficient for early boot
> initialization stuff I am not sure the amount of code is really
> justified by that usecase alone. Any runtime enabled parallelized work
> really have to care about the rest of the system. The last thing you
> really want to see is to make a highly utilized system overloaded just
> because of some optimization. And I do not see how can you achive that
> with a limit on the number of paralelization threads.

That's fair, I'll see what I can do in the next version.

> 
>> For resource isolation, I'll experiment with moving ktask threads into and
>> out of the cgroup of the calling thread.
>>
>> Do any resources not covered by cgroup come to mind?  I'm trying to think if
>> I've left anything out.
> 
> This is mostly about cpu so dealing with the cpu cgroup controller
> should do the work.

Ok, thanks.  Luckily cgroup v2's cpu controller was recently merged.

> 
> [...]
> 
>> Anyway, I think scalability bottlenecks should be weighed with the rest of
>> this.  It seems wrong that the kernel should always assume that one thread
>> is enough to free all of a process's memory or evict all the pages of a file
>> system no matter how much work there is to do.
> 
> Well, this will be always a double edge sword. Sure if you have spare
> cycles (whatever that means) than using them is really nice. But the
> last thing you really want is to turn an optimization into an
> utilization nightmare where few processes dominant the whole machine
> even though they could be easily contained normally inside a single
> execution context. >
> Your work targets larger machines and I understand that you are mainly
> focused on a single large workload running on that machine but there are
> many others running with many smaller workloads which would like to be
> independent. Not everything is a large DB running on a large HW.

Well of course, yes, but the struct page initialization stuff benefits 
any large-memory machine (9x faster on a 2-socket machine!) and the 
(forthcoming) page freeing parallelization will similarly benefit a 
variety of workloads.

Anyway, I'll put more controls in and see where I get.  Thanks for the 
feedback.

Daniel