Re: [PATCH] x86/mce: Dynamically size space for machine check records

Naik, Avadhut
2024-02-29T06:42:44Z

Hi,

On 2/28/2024 17:14, Tony Luck wrote:
> Systems with a large number of CPUs may generate a large
> number of machine check records when things go seriously
> wrong. But Linux has a fixed buffer that can only capture
> a few dozen errors.
>
> Allocate space based on the number of CPUs (with a minimum
> value based on the historical fixed buffer that could store
> 80 records).
>
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
>
> Discussion earlier concluded with the realization that it is
> safe to dynamically allocate the mce_evt_pool at boot time.
> So here's a patch to do that. Scaling algorithm here is a
> simple linear "4 records per possible CPU" with a minimum
> of 80 to match the legacy behavior. I'm open to other
> suggestions.
>
> Note that I threw in a "+1" to the return from ilog2() when
> calling gen_pool_create(). From reading code, and running
> some tests, it appears that the min_alloc_order argument
> needs to be large enough to allocate one of the mce_evt_llist
> structures.
>
> Some other gen_pool users in Linux may also need this "+1".
>

Somewhat confused here. Weren't we also exploring ways to avoid
duplicate records from being added to the genpool? Has something
changed in that regard?

> arch/x86/kernel/cpu/mce/genpool.c | 22 ++++++++++++++++------
> 1 file changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/mce/genpool.c b/arch/x86/kernel/cpu/mce/genpool.c
> index fbe8b61c3413..a1f0a8f29cf5 100644
> --- a/arch/x86/kernel/cpu/mce/genpool.c
> +++ b/arch/x86/kernel/cpu/mce/genpool.c
> @@ -16,14 +16,13 @@
> * used to save error information organized in a lock-less list.
> *
> * This memory pool is only to be used to save MCE records in MCE context.
> - * MCE events are rare, so a fixed size memory pool should be enough. Use
> - * 2 pages to save MCE events for now (~80 MCE records at most).
> + * MCE events are rare, so a fixed size memory pool should be enough.
> + * Allocate on a sliding scale based on number of CPUs.
> */
> -#define MCE_POOLSZ (2 * PAGE_SIZE)
> +#define MCE_MIN_ENTRIES 80
>
> static struct gen_pool *mce_evt_pool;
> static LLIST_HEAD(mce_event_llist);
> -static char gen_pool_buf[MCE_POOLSZ];
>
> /*
> * Compare the record "t" with each of the records on list "l" to see if
> @@ -118,14 +117,25 @@ int mce_gen_pool_add(struct mce *mce)
>
> static int mce_gen_pool_create(void)
> {
> + int mce_numrecords, mce_poolsz;
> struct gen_pool *tmpp;
> int ret = -ENOMEM;
> + void *mce_pool;
> + int order;
>
> - tmpp = gen_pool_create(ilog2(sizeof(struct mce_evt_llist)), -1);
> + order = ilog2(sizeof(struct mce_evt_llist)) + 1;
> + tmpp = gen_pool_create(order, -1);
> if (!tmpp)
> goto out;
>
> - ret = gen_pool_add(tmpp, (unsigned long)gen_pool_buf, MCE_POOLSZ, -1);
> + mce_numrecords = max(80, num_possible_cpus() * 4);
> + mce_poolsz = mce_numrecords * (1 << order);
> + mce_pool = kmalloc(mce_poolsz, GFP_KERNEL);

To err on the side of caution, wouldn't kzalloc() be a safer choice here?

--
Thanks,
Avadhut Naik