Re: [PATCH v4] irqchip/gicv3-its: Avoid memory over allocation for ITEs

From: Shanker Donthineni
Date: Tue Jun 20 2017 - 21:22:49 EST

Hi Marc,

On 05/06/2017 06:25 AM, Marc Zyngier wrote:
> On Fri, May 05 2017 at 11:04:22 pm BST, Shanker Donthineni <shankerd@xxxxxxxxxxxxxx> wrote:
>> Hi Marc,
>>
>>
>> On 05/02/2017 11:16 AM, Marc Zyngier wrote:
>>> On Sun, Apr 30 2017 at 3:36:15 pm BST, Shanker Donthineni <shankerd@xxxxxxxxxxxxxx> wrote:
>>>> We are always allocating extra 255Bytes of memory to handle ITE
>>>> physical address alignment requirement. The kmalloc() satisfies
>>>> the ITE alignment since the ITS driver is requesting a minimum
>>>> size of ITS_ITT_ALIGN bytes.
>>>>
>>>> Let's try to allocate the exact amount of memory that is required
>>>> for ITEs to avoid wastage.
>>>>
>>>> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
>>>> ---
>>>> Changes:
>>>> v2: removed 'Change-Id: Ia8084189833f2081ff13c392deb5070c46a64038' from commit.
>>>> v3: changed from IITE to ITE.
>>>> v3: removed fallback since kmalloc() guarantees the right alignment.
>>>>
>>>> drivers/irqchip/irq-gic-v3-its.c | 6 +++---
>>>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>>>> index 45ea1933..72e56f03 100644
>>>> --- a/drivers/irqchip/irq-gic-v3-its.c
>>>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>>>> @@ -261,7 +261,6 @@ static struct its_collection *its_build_mapd_cmd(struct its_cmd_block *cmd,
>>>> u8 size = ilog2(desc->its_mapd_cmd.dev->nr_ites);
>>>>
>>>> itt_addr = virt_to_phys(desc->its_mapd_cmd.dev->itt);
>>>> - itt_addr = ALIGN(itt_addr, ITS_ITT_ALIGN);
>>>>
>>>> its_encode_cmd(cmd, GITS_CMD_MAPD);
>>>> its_encode_devid(cmd, desc->its_mapd_cmd.dev->device_id);
>>>> @@ -1329,13 +1328,14 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>>>> */
>>>> nr_ites = max(2UL, roundup_pow_of_two(nvecs));
>>>> sz = nr_ites * its->ite_size;
>>>> - sz = max(sz, ITS_ITT_ALIGN) + ITS_ITT_ALIGN - 1;
>>>> + sz = max(sz, ITS_ITT_ALIGN);
>>>> itt = kzalloc(sz, GFP_KERNEL);
>>>> lpi_map = its_lpi_alloc_chunks(nvecs, &lpi_base, &nr_lpis);
>>>> if (lpi_map)
>>>> col_map = kzalloc(sizeof(*col_map) * nr_lpis, GFP_KERNEL);
>>>>
>>>> - if (!dev || !itt || !lpi_map || !col_map) {
>>>> + if (!dev || !itt || !lpi_map || !col_map ||
>>>> + !IS_ALIGNED(virt_to_phys(itt), ITS_ITT_ALIGN)) {
>>>> kfree(dev);
>>>> kfree(itt);
>>>> kfree(lpi_map);
>>> I'm confused. Either the alignment is guaranteed (and you should
>>> document why it is so), or it is not, and we need to handle the
>>> non-alignment (instead of failing).
>>
>> Sorry for confusion, alignment is guaranteed by kmalloc(), added a
>> check for readability purpose only can be removed.
>
> My question still remains. Where exactly is that alignment guarantee
> documented and enforced? I can't see anything giving that certainty.
>

The internal implementation of kmalloc() uses the slab/slub feature to allocate memory from 2^N size pool. Linux kernel maintains the fixed size of kmem_cache pools to serve the kmalloc(), It allocates minimum size of 128Bytes and maximum size depends on the system configuration and memory availability. In fact SMMUv3 driver has a similar requirement and absolutely there no problem using kmalloc() to meet the address alignment requirement.

Call trace:
kmalloc()
kmalloc_slab() --> convert size to kmem_cache
slab_alloc() ---> allocate 2^N size kmem_cache object

root@null-8cfdf006971f:~# cat /proc/slabinfo | grep kmall
dma-kmalloc-131072 0 0 131072 4 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-65536 0 0 65536 8 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-32768 0 0 32768 16 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-16384 0 0 16384 32 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-8192 0 0 8192 32 4 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-4096 0 0 4096 32 2 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-2048 0 0 2048 32 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-1024 0 0 1024 64 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-512 128 128 512 128 1 : tunables 0 0 0 : slabdata 1 1 0
dma-kmalloc-256 0 0 256 256 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-128 512 512 128 512 1 : tunables 0 0 0 : slabdata 1 1 0
kmalloc-131072 4 4 131072 4 8 : tunables 0 0 0 : slabdata 1 1 0
kmalloc-65536 376 376 65536 8 8 : tunables 0 0 0 : slabdata 47 47 0
kmalloc-32768 320 320 32768 16 8 : tunables 0 0 0 : slabdata 20 20 0
kmalloc-16384 5248 5248 16384 32 8 : tunables 0 0 0 : slabdata 164 164 0
kmalloc-8192 2176 2176 8192 32 4 : tunables 0 0 0 : slabdata 68 68 0
kmalloc-4096 4452 4576 4096 32 2 : tunables 0 0 0 : slabdata 143 143 0
kmalloc-2048 4416 4416 2048 32 1 : tunables 0 0 0 : slabdata 138 138 0
kmalloc-1024 10048 10176 1024 64 1 : tunables 0 0 0 : slabdata 159 159 0
kmalloc-512 19071 19584 512 128 1 : tunables 0 0 0 : slabdata 153 153 0
kmalloc-256 75873 77312 256 256 1 : tunables 0 0 0 : slabdata 302 302 0
kmalloc-128 82078 85504 128 512 1 : tunables 0 0 0 : slabdata 167 167 0


> I would expect kmalloc to give you something that is cache-line aligned,
> but probably nothing more than that. Now, I'd happily be proven wrong,
> but so far, all I can see is that:
>
> - ARCH_KMALLOC_MINALIGN is defined as ARCH_DMA_MINALIGN
> - ARCH_DMA_MINALIGN is defined as L1_CACHE_BYTES
> - L1_CACHE_BYTES is 128 on arm64, and either 32, 64, or 128 on arm.
>

Kmalloc always allocates memory with size=roundup_pow_of_two(size) and address alignment roundup_pow_of_two(size).

> What am I missing?
>
> Thanks,
>
> M.
>

--
Shanker Donthineni
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.