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Wysocki" Cc: "Rafael J. Wysocki" , Linux PM , LKML , Amit Kucheria , "Pandruvada\, Srinivas" , Rodrigo Vivi , Peter Zijlstra Subject: Re: [PATCH 00/28] PM: QoS: Get rid of unuseful code and rework CPU latency QoS interface In-Reply-To: References: <1654227.8mz0SueHsU@kreacher> <87wo8rjsa4.fsf@riseup.net> Date: Thu, 13 Feb 2020 00:10:05 -0800 Message-ID: <877e0qj4bm.fsf@riseup.net> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="==-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --==-=-= Content-Type: multipart/mixed; boundary="=-=-=" --=-=-= Content-Type: text/plain; charset=utf-8 Content-Disposition: inline "Rafael J. Wysocki" writes: > On Thu, Feb 13, 2020 at 1:16 AM Rafael J. Wysocki wrote: >> >> On Thu, Feb 13, 2020 at 12:31 AM Francisco Jerez wrote: >> > >> > "Rafael J. Wysocki" writes: >> > >> > > Hi All, >> > > >> > > This series of patches is based on the observation that after commit >> > > c3082a674f46 ("PM: QoS: Get rid of unused flags") the only global PM QoS class >> > > in use is PM_QOS_CPU_DMA_LATENCY, but there is still a significant amount of >> > > code dedicated to the handling of global PM QoS classes in general. That code >> > > takes up space and adds overhead in vain, so it is better to get rid of it. >> > > >> > > Moreover, with that unuseful code removed, the interface for adding QoS >> > > requests for CPU latency becomes inelegant and confusing, so it is better to >> > > clean it up. >> > > >> > > Patches [01/28-12/28] do the first part described above, which also includes >> > > some assorted cleanups of the core PM QoS code that doesn't go away. >> > > >> > > Patches [13/28-25/28] rework the CPU latency QoS interface (in the classic >> > > "define stubs, migrate users, change the API proper" manner), patches >> > > [26-27/28] update the general comments and documentation to match the code >> > > after the previous changes and the last one makes the CPU latency QoS depend >> > > on CPU_IDLE (because cpuidle is the only user of its target value today). >> > > >> > > The majority of the patches in this series don't change the functionality of >> > > the code at all (at least not intentionally). >> > > >> > > Please refer to the changelogs of individual patches for details. >> > > >> > > Thanks! >> > >> > Hi Rafael, >> > >> > I believe some of the interfaces removed here could be useful in the >> > near future. >> >> I disagree. >> >> > It goes back to the energy efficiency- (and IGP graphics >> > performance-)improving series I submitted a while ago [1]. It relies on >> > some mechanism for the graphics driver to report an I/O bottleneck to >> > CPUFREQ, allowing it to make a more conservative trade-off between >> > energy efficiency and latency, which can greatly reduce the CPU package >> > energy usage of IO-bound applications (in some graphics benchmarks I've >> > seen it reduced by over 40% on my ICL laptop), and therefore also allows >> > TDP-bound applications to obtain a reciprocal improvement in throughput. >> > >> > I'm not particularly fond of the global PM QoS interfaces TBH, it seems >> > like an excessively blunt hammer to me, so I can very much relate to the >> > purpose of this series. However the finer-grained solution I've >> > implemented has seen some push-back from i915 and CPUFREQ devs due to >> > its complexity, since it relies on task scheduler changes in order to >> > track IO bottlenecks per-process (roughly as suggested by Peter Zijlstra >> > during our previous discussions), pretty much in the spirit of PELT but >> > applied to IO utilization. >> > >> > With that in mind I was hoping we could take advantage of PM QoS as a >> > temporary solution [2], by introducing a global PM QoS class similar but >> > with roughly converse semantics to PM_QOS_CPU_DMA_LATENCY, allowing >> > device drivers to report a *lower* bound on CPU latency beyond which PM >> > shall not bother to reduce latency if doing so would have negative >> > consequences on the energy efficiency and/or parallelism of the system. >> >> So I really don't quite see how that could be responded to, by cpuidle >> say. What exactly do you mean by "reducing latency" in particular? >> >> > Of course one would expect the current PM_QOS_CPU_DMA_LATENCY upper >> > bound to take precedence over the new lower bound in cases where the >> > former is in conflict with the latter. >> >> So that needs to be done on top of this series. >> >> > I can think of several alternatives to that which don't involve >> > temporarily holding off your clean-up, >> >> The cleanup goes in. Please work on top of it. >> >> > but none of them sound particularly exciting: >> > >> > 1/ Use an interface specific to CPUFREQ, pretty much like the one >> > introduced in my original submission [1]. >> >> It uses frequency QoS already today, do you really need something else? >> >> > 2/ Use per-CPU PM QoS, which AFAICT would require the graphics driver >> > to either place a request on every CPU of the system (which would >> > cause a frequent operation to have O(N) complexity on the number of >> > CPUs on the system), or play a cat-and-mouse game with the task >> > scheduler. >> >> That's in place already too in the form of device PM QoS; see >> drivers/base/power/qos.c. >> >> > 3/ Add a new global PM QoS mechanism roughly duplicating the >> > cpu_latency_qos_* interfaces introduced in this series. Drop your >> > change making this available to CPU IDLE only. >> >> It sounds like you really want performance for energy efficiency and >> CPU latency has a little to do with that. >> >> > 3/ Go straight to a scheduling-based approach, which is likely to >> > greatly increase the review effort required to upstream this >> > feature. (Peter might disagree though?) >> >> Are you familiar with the utilization clamps mechanism? > > And BTW, posting patches as RFC is fine even if they have not been > tested. At least you let people know that you work on something this > way, so if they work on changes in the same area, they may take that > into consideration. > Sure, that was going to be the first RFC. > Also if there are objections to your proposal, you may save quite a > bit of time by sending it early. > > It is unfortunate that this series has clashed with the changes that > you were about to propose, but in this particular case in my view it > is better to clean up things and start over. > Luckily it doesn't clash with the second RFC I was meaning to send, maybe we should just skip the first? Or maybe it's valuable as a curiosity anyway? > Thanks! --=-=-=-- --==-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEAREIAB0WIQST8OekYz69PM20/4aDmTidfVK/WwUCXkUEXQAKCRCDmTidfVK/ W4pAAP9vmbs26tAnKsYJ+S4uoe86MagT3K6/cDduW1R8rKZwdQEAoMhECi5Ar//t Idku9y/+hBdBSWgH1os3DRQNWpLrdjE= =zHCk -----END PGP SIGNATURE----- --==-=-=--