netdev - Re: [RFC 1/2] pm: Introduce QoS requests per CPU

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Date:	Wed, 26 Mar 2014 13:36:37 -0400
From:	Jeremy Eder <jeder@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Amir Vadai <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>, linux-pm@...r.kernel.org,
	netdev@...r.kernel.org, Pavel Machek <pavel@....cz>,
	Len Brown <len.brown@...el.com>, yuvali@...lanox.com,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>, idos@...lanox.com
Subject: Re: [RFC 1/2] pm: Introduce QoS requests per CPU

On 140325 19:44:53, Rafael J. Wysocki wrote:
> On Tuesday, March 25, 2014 03:18:24 PM Amir Vadai wrote:
> > Extend the current pm_qos_request API - to have pm_qos_request per core.
> > When a global request is added, it is added under the global plist.
> > When a core specific request is added, it is added to the core specific
> > list.
> > core number is saved in the request and later modify/delete operations
> > are using it to access the right list.
> > 
> > When a cpu specific request is added/removed/updated, the target value
> > of the specific core is recalculated to be the min/max (according to the
> > constrain type) value of all the global and the cpu specific
> > constraints.
> > 
> > If a global request is added/removed/updated, the target values of all
> > the cpu's are recalculated.
> > 
> > During initialization, before the cpu specific data structures are
> > allocated and initialized, only global target value is begin used.
> 
> I have to review this in detail (which rather won't be possible before
> the next week), but in principle I don't really like it, because it
> assumes that its users will know what's going to run on which CPU cores
> and I'm not sure where that knowledge is going to come from.

Hi guys,

I think busy_poll can accomplish the basic goals of this patch
set.  Stop drops due to c-state transition latency.  Get into more performant
c-states only on active cores with SO_BUSY_POLL or the sysctl.

Whether it's system-wide or per-cpu, cpu_dma_latency wastes power and
worse, it's a static thing.  We need adaptable power management for the
general case.  I guess that might look like power-aware scheduling, or wiring
menu.c to incorporate hints from drivers/userspace.

cpu_dma_latency reduces TDP headroom because non-active cores are in
unnecessarily high c-states, reduces the amount of turbo boost you can have,
and thus reduces performance of (i.e.) low-thread-count workloads.

busy_poll has another positive side-effect; it's even more granular (thus
more power friendly) than the percpu idea:  it will only affect cores that
have active sockets on them.  When the sockets aren't active, the core can
settle into a deep c-state, and possibly the socket can settle into a deeper
package c-state.  There's some data in the blog post that Jesper sent.

I also want to mention that this "class" of issue is not particularly
related to networking.
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