From: Marc Gonzalez <marc.w.gonzalez@free.fr>
To: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>,
Peter Zijlstra <peterz@infradead.org>,
Robin Murphy <robin.murphy@arm.com>,
Vineet Gupta <Vineet.Gupta1@synopsys.com>,
Dmitry Torokhov <dmitry.torokhov@gmail.com>,
Rafael Wysocki <rjw@rjwysocki.net>,
LKML <linux-kernel@vger.kernel.org>,
Bjorn Andersson <bjorn.andersson@linaro.org>,
Russell King <rmk+kernel@armlinux.org.uk>,
Mark Brown <broonie@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Tejun Heo <tj@kernel.org>,
arcml <linux-snps-arc@lists.infradead.org>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()
Date: Wed, 18 Dec 2019 16:40:22 +0100 [thread overview]
Message-ID: <bff4c4ad-de8f-7229-1d16-7ea67e066f65@free.fr> (raw)
In-Reply-To: <CY4PR1201MB012059FF6735C3EDB7E1F8E9A1530@CY4PR1201MB0120.namprd12.prod.outlook.com>
On 18/12/2019 15:20, Alexey Brodkin wrote:
> On 17/12/2019 16:30, Marc Gonzalez wrote:
>
>> Commit a66d972465d15 ("devres: Align data[] to ARCH_KMALLOC_MINALIGN")
>> increased the alignment of devres.data unconditionally.
>>
>> Some platforms have very strict alignment requirements for DMA-safe
>> addresses, e.g. 128 bytes on arm64. There, struct devres amounts to:
>> 3 pointers + pad_to_128 + data + pad_to_256
>> i.e. ~220 bytes of padding.
>
> Could you please elaborate a bit on mentioned paddings?
> I may understand the first one for 128 bytes but where does the
> second one for 256 bytes come from?
Sure thing.
struct devres {
struct devres_node node;
u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
};
struct devres_node = 3 pointers
kmalloc dishes out memory in multiples of ARCH_KMALLOC_MINALIGN bytes.
On arm64, ARCH_KMALLOC_MINALIGN = 128
(Everything written below assumes ARCH_KMALLOC_MINALIGN = 128)
In alloc_dr() we request sizeof(struct devres) + sizeof(data) from kmalloc.
sizeof(struct devres) = 128 because of the alignment directive.
I.e. the 'data' field is automatically padded to 128 by the compiler.
For most devm allocs (non-devm_kmalloc allocs), data is just 1 or 2 pointers.
So kmalloc(128 + 16) allocates 256 bytes.
>> Let's enforce the alignment only for devm_kmalloc().
>
> Ok so for devm_kmalloc() we don't change anything, right?
> We still add the same padding before real data array.
(My commit message probably requires improvement & refining.)
Yes, the objective of my patch is to keep the same behavior for
devm_kmalloc() while reverting to the old behavior for all other
uses of struct devres.
>> I had not been aware that dynamic allocation granularity on arm64 was
>> 128 bytes. This means there's a lot of waste on small allocations.
>
> Now probably I'm missing something but when do you expect to save something?
> If those smaller allocations are done with devm_kmalloc() you aren't
> saving anything.
With my patch, a "non-kmalloc" struct devres would take 128 bytes, instead
of 256.
>> I suppose there's no easy solution, though.
>
> Right! It took a while till I was able to propose something
> people [almost silently] agreed with.
I meant the wider subject of dynamic allocation granularity.
The 128-byte requirement is only for DMA. Some (most?) uses of kmalloc
are not for DMA. If the user could provide a flag ("this is to be used
for DMA") we could save lots of memory for small non-DMA allocs.
>> +#define DEVM_KMALLOC_PADDING_SIZE \
>> + (ARCH_KMALLOC_MINALIGN - sizeof(struct devres) % ARCH_KMALLOC_MINALIGN)
>
> Even given your update with:
> ------------------------------->8--------------------------------
> #define DEVM_KMALLOC_PADDING_SIZE \
> ((ARCH_KMALLOC_MINALIGN - sizeof(struct devres)) % ARCH_KMALLOC_MINALIGN)
> ------------------------------->8--------------------------------
> I don't think I understand why do you need that "% ARCH_KMALLOC_MINALIGN" part?
To handle the case where sizeof(struct devres) > ARCH_KMALLOC_MINALIGN
e.g ARCH_KMALLOC_MINALIGN = 8 and sizeof(struct devres) = 12
>> + /* Add enough padding to provide a DMA-safe address */
>> + size += DEVM_KMALLOC_PADDING_SIZE;
>
> This implementation gets ugly and potentially will lead to problems later
> when people will start changing code here. Compared to that initially aligned by
> the compiler dr->data looks much more foolproof.
Yes, it's better to let the compiler handle the padding... But, we don't
want any padding in the non-devm_kmalloc use-case.
We could add a pointer to the data field, but arches with small ARCH_KMALLOC_MINALIGN
will have to pay the size increase, which doesn't seem fair to them (x86, amd64).
>> @@ -822,7 +825,7 @@ void * devm_kmalloc(struct device *dev, size_t size, gfp_t gfp)
>> */
>> set_node_dbginfo(&dr->node, "devm_kzalloc_release", size);
>> devres_add(dev, dr->data);
>> - return dr->data;
>> + return dr->data + DEVM_KMALLOC_PADDING_SIZE;
>
> Ditto. But first I'd like to understand what are you trying to really do
> with your change and then we'll see if there could be any better implementation.
Basically, every call to devres_alloc() or devm_add_action() allocates
256 bytes instead of 128. A typical arm64 system will call these
thousands of times during driver probe.
Regards.
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next prev parent reply other threads:[~2019-12-18 15:41 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <74ae22cd-08c1-d846-3e1d-cbc38db87442@free.fr>
2019-12-18 14:20 ` [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc() Alexey Brodkin
2019-12-18 15:40 ` Marc Gonzalez [this message]
[not found] ` <bf020a68-00fd-2bb7-c3b6-00f5befa293a@free.fr>
[not found] ` <20191220140655.GN2827@hirez.programming.kicks-ass.net>
[not found] ` <9be1d523-e92c-836b-b79d-37e880d092a0@arm.com>
2019-12-20 19:32 ` Alexey Brodkin
2019-12-20 20:23 ` Peter Zijlstra
2019-12-20 21:02 ` Alexey Brodkin
2019-12-20 21:47 ` Dmitry Torokhov
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