Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 26 Jan 2023 18:20:26 -0800 | From | Chris Goldsworthy <> | Subject | Re: [RFC] mm: Allow ZONE_DMA32 to be disabled via kernel command line |
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On Thu, Jan 26, 2023 at 07:15:26PM +0000, Robin Murphy wrote: > On 2023-01-26 16:43, Georgi Djakov wrote: > >From: Chris Goldsworthy <quic_cgoldswo@quicinc.com> > > > >It's useful to have an option to disable the ZONE_DMA32 during boot as > >CONFIG_ZONE_DMA32 is by default enabled (on multiplatform kernels for > >example). There are platforms that do not use this zone and in some high > >memory pressure scenarios this would help on easing kswapd (to leave file > >backed memory intact / unreclaimed). When the ZONE_DMA32 is enabled on > >these platforms - kswapd is woken up more easily and drains the file cache > >which leads to some performance issues. > > > >Signed-off-by: Chris Goldsworthy <quic_cgoldswo@quicinc.com> > >[georgi: updated commit text] > >Signed-off-by: Georgi Djakov <quic_c_gdjako@quicinc.com> > >--- > >The main question here is whether we can have a kernel command line > >option to disable CONFIG_ZONE_DMA32 during boot (at least on arm64). > >I can imagine this being useful also for Linux distros. > > FWIW I'd say that "disabled" and "left empty then awkwardly tiptoed around > in a few places" are very different notions... > > However, I'm just going to take a step back and read the commit message a > few more times... Given what it claims, I can't help but ask why wouldn't we > want a parameter to control kswapd's behaviour and address that issue > directly, rather than a massive hammer that breaks everyone allocating > explicitly or implicitly with __GFP_DMA32 (especially on systems where it > doesn't normally matter because all memory is below 4GB anyway), just to > achieve one rather niche side-effect? > > Thanks, > Robin.
Hi Robin,
The commit text doesn't spell out the scenario we want to avoid, so I will do that for clarity. We use a kernel binary compiled for us, and by default has CONFIG_ZONE_DMA32 (and it can't be disabled for now as another party needs it). Our higher-end SoCs are usually used with 8-12 GB of DDR, so using a 12 GB device as an example, we would have 8 GB of ZONE_NORMAL memory and 4 GB of ZONE_MOVABLE memory with the feature, and 4 GB of ZONE_DMA32, 4 GB of ZONE_NORMAL and 4 GB of ZONE_MOVABLE otherwise.
Without the feature enabled, consider a GFP_KERNEL allocation that causes a low watermark beach in ZONE_NORMAL, such that such that ZONE_DMA32 is almost full. This will cause kswapd to start reclaiming memory, despite the fact that that we might have gigabytes of free memory in ZONE_DMA32 that can be used by anyone (since GFP_MOVABLE and GFP_NORMAL can fall back to using ZONE_DMA32).
So, fleshing out your suggestion to make it concrete for our case, we would want to stop kswapd from doing reclaim on ZONE_NORMAL watermark breaches when ZONE_DMA32 is present (since anything targeting ZONE_NORMAL can fall back to ZONE_DMA32).
Thanks,
Chris.
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