RFC: Faster memtest (possibly bypassing data cache)

[Marc Gonzalez]

On 05/07/2023 17:41, Marc Gonzalez wrote:

> Hello,
> 
> When dealing with a few million devices (x86 and arm64),
> it is statistically expected to have "a few" devices with
> at least one bad RAM cell. (How many?)
> 
> For one particular model, we've determined that ~0.1% have
> at least one bad RAM cell (ergo, a few thousand devices).
> 
> I've been wondering if someone more experienced knows:
> Are these RAM cells bad from the start, or do they become bad
> with time? (I assume both failure modes exist.)
> 
> Once the first bad cell is detected, is it more likely
> to detect other bad cells as time goes by?
> In other words, what are the failure modes of ageing RAM?
> 
> 
> Closing the HW tangent, focusing on the SW side of things:
> 
> Since these bad RAM cells wreak havoc for the device's user,
> especially with ASLR (different stuff crashes across reboots),
> I've been experimenting with mm/memtest.c as a first line
> of defense against bad RAM cells.
> 
> However, I have a run into a few issues.
> 
> Even though early_memtest is called, well... early, memory has
> already been mapped as regular *cached* memory.
> 
> This means that when we test an area smaller than L3 cache, we're
> not even hitting RAM, we're just testing the cache hierarchy.
> I suppose it /might/ make sense to test the cache hierarchy,
> as it could(?) have errors as well?
> However, I suspect defects in cache are much more rare
> (and thus detection might not be worth the added run-time).
> 
> On x86, I ran a few tests using SIMD non-temporal stores
> (to bypass the cache on stores), and got 30% reduction in run-time.
> (Minimal run-time is critical for being able to deploy the code
> to millions of devices for the benefit of a few thousand users.)
> AFAIK, there are no non-temporal loads, the normal loads probably
> thrashed the data cache.
> 
> I was hoping to be able to test a different implementation:
> 
> When we enter early_memtest(), we remap [start, end]
> as UC (or maybe WC?) so as to entirely bypass the cache.
> We read/write using the largest size available for stores/loads,
> e.g. entire cache lines on recent x86 HW.
> Then when we leave, we remap as was done originally.
> 
> Is that possible?
> 
> Hopefully, the other cores are not started at this point?
> (Otherwise this whole charade would be pointless.)
> 
> To summarize: is it possible to tweak memtest to make it
> run faster while testing RAM in all cases?

Hello again,

I had a short chat with Robin on IRC.

He said trying to bypass the cache altogether was a bad idea(TM)
performance-wise. Do others agree with this assessment? :)

Would like to read people's thoughts about the whole thing.

What is the kernel API to flush a kernel memory range to memory?

  int flush_cache_to_memory(void *va_start, void *va_end);

On aarch64, I would test LDNP/STNP. Possibly also LD4/ST4.

Regards,

Marc