linux-kernel - Re: [RFC PATCH v4 01/13] ktask: add documentation

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Message-ID: <20181106022747.dmtq24pvulcnv3lc@ca-dmjordan1.us.oracle.com>
Date:   Mon, 5 Nov 2018 18:27:47 -0800
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>, linux-mm@...ck.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        aarcange@...hat.com, aaron.lu@...el.com, akpm@...ux-foundation.org,
        alex.williamson@...hat.com, bsd@...hat.com,
        darrick.wong@...cle.com, dave.hansen@...ux.intel.com,
        jgg@...lanox.com, jwadams@...gle.com, jiangshanlai@...il.com,
        mhocko@...nel.org, mike.kravetz@...cle.com,
        Pavel.Tatashin@...rosoft.com, prasad.singamsetty@...cle.com,
        steven.sistare@...cle.com, tim.c.chen@...el.com, tj@...nel.org,
        vbabka@...e.cz
Subject: Re: [RFC PATCH v4 01/13] ktask: add documentation

On Mon, Nov 05, 2018 at 01:19:50PM -0800, Randy Dunlap wrote:
> On 11/5/18 8:55 AM, Daniel Jordan wrote:
> 
> Hi,
> 
> > +Resource Limits
> > +===============
> > +
> > +ktask has resource limits on the number of work items it sends to workqueue.
> 
>                                                                   to a workqueue.
> or:                                                               to workqueues.

Ok, I'll do "to workqueues" since ktask uses two internally (NUMA-aware and
non-NUMA-aware).

> 
> > +In ktask, a workqueue item is a thread that runs chunks of the task until the
> > +task is finished.
> > +
> > +These limits support the different ways ktask uses workqueues:
> > + - ktask_run to run threads on the calling thread's node.
> > + - ktask_run_numa to run threads on the node(s) specified.
> > + - ktask_run_numa with nid=NUMA_NO_NODE to run threads on any node in the
> > +   system.
> > +
> > +To support these different ways of queueing work while maintaining an efficient
> > +concurrency level, we need both system-wide and per-node limits on the number
> 
> I would prefer to refer to ktask as ktask instead of "we", so
> s/we need/ktask needs/

Good idea, I'll change it.

> > +of threads.  Without per-node limits, a node might become oversubscribed
> > +despite ktask staying within the system-wide limit, and without a system-wide
> > +limit, we can't properly account for work that can run on any node.
> 
> s/we/ktask/

Ok.

> > +
> > +The system-wide limit is based on the total number of CPUs, and the per-node
> > +limit on the CPU count for each node.  A per-node work item counts against the
> > +system-wide limit.  Workqueue's max_active can't accommodate both types of
> > +limit, no matter how many workqueues are used, so ktask implements its own.
> > +
> > +If a per-node limit is reached, the work item is allowed to run anywhere on the
> > +machine to avoid overwhelming the node.  If the global limit is also reached,
> > +ktask won't queue additional work items until we fall below the limit again.
> 
> s/we fall/ktask falls/
> or s/we fall/it falls/

'ktask.'  Will change.

> > +
> > +These limits apply only to workqueue items--that is, helper threads beyond the
> > +one starting the task.  That way, one thread per task is always allowed to run.
> 
> 
> thanks.

Appreciate the feedback!