Re: [PATCH v7 31/49] x86/resctrl: Remove the limit on the number of CLOSID

Reinette Chatre
2025-03-07T04:40:09Z

Hi James,

On 2/28/25 11:58 AM, James Morse wrote:
> From: Amit Singh Tomar <amitsinght@marvell.com>
>
> Resctrl allocates and finds free CLOSID values using the bits of a u32.
> This restricts the number of control groups that can be created by
> user-space.
>
> MPAM has an architectural limit of 2^16 CLOSID values, Intel x86 could
> be extended beyond 32 values. There is at least one MPAM platform which
> supports more than 32 CLOSID values.
>
> Replace the fixed size bitmap with calls to the bitmap API to allocate
> an array of a sufficient size.
>
> ffs() returns '1' for bit 0, hence the existing code subtracts 1 from
> the index to get the CLOSID value. find_first_bit() returns the bit
> number which does not need adjusting.
>
> Signed-off-by: Amit Singh Tomar <amitsinght@marvell.com>
> [ morse: fixed the off-by-one in the allocator and the wrong
> not-found value. Removed the limit. Rephrase the commit message. ]
> Signed-off-by: James Morse <james.morse@arm.com>

...

> @@ -3071,6 +3085,7 @@ static void rdt_kill_sb(struct super_block *sb)
> resctrl_arch_disable_alloc();
> if (resctrl_arch_mon_capable())
> resctrl_arch_disable_mon();
> + closid_exit();
> resctrl_mounted = false;
> kernfs_kill_sb(sb);
> mutex_unlock(&rdtgroup_mutex);

Above is the new change in this patch ... I am trying to understand the choice
in ordering since I expect that freeing resources is done in opposite
order from what it was allocated. I thus expected it to be before
schemata_list_destroy() but it is instead done as the last thing before removing
the superblock.

The changelog does not mention dependencies that need to be kept in mind.
I thought that there may be something going on with open files ... for
example if user kept "bit_usage" (that calls closid_allocated() that
depends on the closid_free_map) but a quick test confirmed that
if a file is open then an attempt to unmount will get a resource
busy error. So rdt_kill_sb() will not even start while a file is open.
Specifically, user sees a "umount: /sys/fs/resctrl: target is busy"

What am I missing?

Reinette