Messages in this thread | | | Subject | Re: [PATCH] Doc: lockdep: add information about performance impact | From | Lukasz Luba <> | Date | Wed, 10 Oct 2018 13:49:28 +0200 |
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Hi Robin,
On 10/10/2018 12:31 PM, Robin Murphy wrote: > On 09/10/18 17:06, Lukasz Luba wrote: >> Hi Peter, >> >> On 10/09/2018 05:43 PM, Peter Zijlstra wrote: >>> On Tue, Oct 09, 2018 at 05:39:27PM +0200, Lukasz Luba wrote: >>>> This patch add some warning related to performance drop. >>>> It should be mentioned that this is not for free >>>> and the platfrom resources (cache, bus interconnect, etc.) >>>> will be used more frequently. >>> >>> To me this reads a bit like: water is wet. >>> >>> Is this really needed? >>> >>> >> Well, it would be good to know what is the performance drop >> (10% or 20% or x3 times) when you are enabling different debug options. >> I have spent some time analyzing these cache and bus strange behavior. >> Now the developers would know that LOCKDEP might cause constant trashing >> of your cache in some use cases. > > Fair enough, but this is the wrong place for that. Anyone who's got as > far as reading how the internals of lockdep work can probably already > figure out that that brings a non-trivial overhead, whereas Joe > Developer wondering why his kernel is slow seems unlikely to happen > across this document by chance. And the people shipping devices with > PROVE_LOCKING enabled because it happened to mask some tricky bug, well, > they know what they did ;) > > If you want to highlight to unwitting users that a tweaking a particular > config knob has a significant performance hit, at least put the warning > next to said knob, i.e. in the Kconfig help. For an example, DMA debug > comes to mind. > > Robin. > > Thank for the hint. I agree, it would not be easy to find in this location. I've found it during porting EAS where the PROVE_LOCKING is used for arm and arm64 http://linux-arm.org/git?p=linux-power.git;a=blobdiff;f=arch/arm/configs/multi_v7_defconfig;h=e83c6e5bcfff97f8e9aacc4f2086ed506df63e59;hp=2721877d5a11f7fc41b8eb42d040ccde10eb0f3e;hb=737c15bf13632504e94509f0b9508122c664f3f2;hpb=288ae1d294d314b0aad5e0471605089da0336a77
I just wanted to give some rough performance impact measurements on a real platform.
Regards, Lukasz
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