Re: [PATCH v3] mlx4_core: allocate ICM memory in page size chunks

From: Qing Huang
Date: Tue May 22 2018 - 20:48:11 EST



On 5/22/2018 8:33 AM, Tariq Toukan wrote:


On 18/05/2018 12:45 AM, Qing Huang wrote:


On 5/17/2018 2:14 PM, Eric Dumazet wrote:
On 05/17/2018 01:53 PM, Qing Huang wrote:
When a system is under memory presure (high usage with fragments),
the original 256KB ICM chunk allocations will likely trigger kernel
memory management to enter slow path doing memory compact/migration
ops in order to complete high order memory allocations.

When that happens, user processes calling uverb APIs may get stuck
for more than 120s easily even though there are a lot of free pages
in smaller chunks available in the system.

Syslog:
...
Dec 10 09:04:51 slcc03db02 kernel: [397078.572732] INFO: task
oracle_205573_e:205573 blocked for more than 120 seconds.
...

NACK on this patch.

You have been asked repeatedly to use kvmalloc()

This is not a minor suggestion.

Take a look athttps://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d8c13f2271ec5178c52fbde072ec7b562651ed9d

Would you please take a look at how table->icm is being used in the mlx4 driver? It's a meta data used for individual pointer variable referencing,
not as data frag or in/out buffer. It has no need for contiguous phy. memory.

Thanks.


NACK.

This would cause a degradation when iterating the entries of table->icm.
For example, in mlx4_table_get_range.
E.g.
int mlx4_table_get_range(struct mlx4_dev *dev, struct mlx4_icm_table *table,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ u32 start, u32 end)
{
ÂÂÂÂÂÂÂ int inc = MLX4_TABLE_CHUNK_SIZE / table->obj_size;
ÂÂÂÂÂÂÂ int err;
ÂÂÂÂÂÂÂ u32 i;

ÂÂÂÂÂÂÂ for (i = start; i <= end; i += inc) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ err = mlx4_table_get(dev, table, i);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (err)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto fail;
ÂÂÂÂÂÂÂ }

ÂÂÂÂÂÂÂ return 0;
...
}

E.g. mtt obj is 8 bytes, so a 4KB ICM block would have 512 mtt objects. So you will have to allocate
more 512 mtt objects in order to have table->icm pointer to increment by 1 to fetch next pointer
value. So 256K mtt objects are needed in order to traverse table->icm pointer across a page boundary
in the call stacks.

Considering mlx4_table_get_range() is only used in control path, there is no significant gain by using kvzalloc
vs. vzalloc for table->icm.

Anyway, if a user makes sure mlx4 driver to be loaded very early and doesn't remove and reload it afterwards,
we should have enough (and not wasting) contiguous phy mem for table->icm allocation. I will use kvzalloc to
replace vzalloc and send a V4 patch.

Thanks,
Qing



Thanks,
Tariq

And you'll understand some people care about this.

Strongly.

Thanks.


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