linux-kernel - Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

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Message-ID: <CAL_JsqL84LvUrNy095E_sC2UJnB3SFBgsw6wjOKmFM249BHMOA@mail.gmail.com>
Date:   Tue, 14 Apr 2020 10:00:36 -0500
From:   Rob Herring <robh@...nel.org>
To:     Jon Hunter <jonathanh@...dia.com>,
        Karol Herbst <karolherbst@...il.com>
Cc:     devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-tegra <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

+Karol

On Mon, Jan 13, 2020 at 5:12 AM Jon Hunter <jonathanh@...dia.com> wrote:
>
>
> On 10/01/2020 23:50, Rob Herring wrote:
> > On Tue, Jan 7, 2020 at 4:22 AM Jon Hunter <jonathanh@...dia.com> wrote:
> >>
> >> Hi Rob,
> >>
> >> On 11/12/2019 23:23, Rob Herring wrote:
> >>> The phandle cache was added to speed up of_find_node_by_phandle() by
> >>> avoiding walking the whole DT to find a matching phandle. The
> >>> implementation has several shortcomings:
> >>>
> >>>   - The cache is designed to work on a linear set of phandle values.
> >>>     This is true for dtc generated DTs, but not for other cases such as
> >>>     Power.
> >>>   - The cache isn't enabled until of_core_init() and a typical system
> >>>     may see hundreds of calls to of_find_node_by_phandle() before that
> >>>     point.
> >>>   - The cache is freed and re-allocated when the number of phandles
> >>>     changes.
> >>>   - It takes a raw spinlock around a memory allocation which breaks on
> >>>     RT.
> >>>
> >>> Change the implementation to a fixed size and use hash_32() as the
> >>> cache index. This greatly simplifies the implementation. It avoids
> >>> the need for any re-alloc of the cache and taking a reference on nodes
> >>> in the cache. We only have a single source of removing cache entries
> >>> which is of_detach_node().
> >>>
> >>> Using hash_32() removes any assumption on phandle values improving
> >>> the hit rate for non-linear phandle values. The effect on linear values
> >>> using hash_32() is about a 10% collision. The chances of thrashing on
> >>> colliding values seems to be low.
> >>>
> >>> To compare performance, I used a RK3399 board which is a pretty typical
> >>> system. I found that just measuring boot time as done previously is
> >>> noisy and may be impacted by other things. Also bringing up secondary
> >>> cores causes some issues with measuring, so I booted with 'nr_cpus=1'.
> >>> With no caching, calls to of_find_node_by_phandle() take about 20124 us
> >>> for 1248 calls. There's an additional 288 calls before time keeping is
> >>> up. Using the average time per hit/miss with the cache, we can calculate
> >>> these calls to take 690 us (277 hit / 11 miss) with a 128 entry cache
> >>> and 13319 us with no cache or an uninitialized cache.
> >>>
> >>> Comparing the 3 implementations the time spent in
> >>> of_find_node_by_phandle() is:
> >>>
> >>> no cache:        20124 us (+ 13319 us)
> >>> 128 entry cache:  5134 us (+ 690 us)
> >>> current cache:     819 us (+ 13319 us)
> >>>
> >>> We could move the allocation of the cache earlier to improve the
> >>> current cache, but that just further complicates the situation as it
> >>> needs to be after slab is up, so we can't do it when unflattening (which
> >>> uses memblock).
> >>>
> >>> Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> >>> Cc: Michael Ellerman <mpe@...erman.id.au>
> >>> Cc: Segher Boessenkool <segher@...nel.crashing.org>
> >>> Cc: Frank Rowand <frowand.list@...il.com>
> >>> Signed-off-by: Rob Herring <robh@...nel.org>
> >>
> >> With next-20200106 I have noticed a regression on Tegra210 where it
> >> appears that only one of the eMMC devices is being registered. Bisect is
> >> pointing to this patch and reverting on top of next fixes the problem.
> >> That is as far as I have got so far, so if you have any ideas, please
> >> let me know. Unfortunately, there do not appear to be any obvious errors
> >> from the bootlog.
> >
> > I guess that's tegra210-p2371-2180.dts because none of the others have
> > 2 SD hosts enabled. I don't see anything obvious though. Are you doing
> > any runtime mods to the DT?
>
> I have noticed that the bootloader is doing some runtime mods and so
> checking if this is the cause. I will let you know, but most likely,
> seeing as I cannot find anything wrong with this change itself.

Did you figure out the problem here? Karol sees a similar problem on
Tegra210 with the gpu node regulator.

It looks like /external-memory-controller@...1b000 has a duplicate
phandle. Comparing the dtb in the filesystem with what the kernel
gets, that node is added by the bootloader. So the bootloader is
definitely creating a broken dtb.

Rob