[PATCH] Add a counter and debug printf to change the latency inside tlb_sync

[Jessica Clarke]

On 10 Jan 2025, at 02:01, Chao-ying Fu <icebergfu at gmail.com> wrote:
> 
> On Thu, Jan 9, 2025 at 4:52 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>> 
>> On 10 Jan 2025, at 00:03, Chao-ying Fu <icebergfu at gmail.com> wrote:
>>> 
>>> The tlb_sync loop is slowed down by variable latency to avoid
>>> livelock on some platforms.
>> 
>> I don’t understand “by variable latency”. It doesn’t do anything but
>> print.
> 
> This patch serves one purpose to slow down by calculating count % 20
> and call the debug printf that may not actually print to the console.
> Also, we can really use this patch to debug if the tlb_sync loop never
> exits or runs too long.

Yeah no. Hard NAK. That’s what pause is for (or wrs.[sn]to).

Jess

>>> ---
>>> lib/sbi/sbi_tlb.c | 6 ++++++
>>> 1 file changed, 6 insertions(+)
>>> 
>>> diff --git a/lib/sbi/sbi_tlb.c b/lib/sbi/sbi_tlb.c
>>> index 01b31f4..d46b95c 100644
>>> --- a/lib/sbi/sbi_tlb.c
>>> +++ b/lib/sbi/sbi_tlb.c
>>> @@ -247,6 +247,7 @@ static void tlb_process(struct sbi_scratch *scratch)
>>> 
>>> static void tlb_sync(struct sbi_scratch *scratch)
>>> {
>>> + int count = 0;
>>> atomic_t *tlb_sync =
>>> sbi_scratch_offset_ptr(scratch, tlb_sync_off);
>>> 
>>> @@ -256,6 +257,11 @@ static void tlb_sync(struct sbi_scratch *scratch)
>>> * consume fifo requests to avoid deadlock.
>>> */
>>> tlb_process_once(scratch);
>>> + count++;
>> 
>> Technically this could overflow, and count is signed, so UB...
> 
> We can change to unsigned int.
> 
>> 
>>> + if ((count % 20) == 0) {
>>> + sbi_dprintf("tlb_sync loop count = %d value = %ld\n",
>>> +    count, atomic_read(tlb_sync));
>> 
>> What value does this add? 20 isn’t that big...
> 
> This is to slow down the loop more frequently, compared to 1000, for example.
> 
>> 
>> If your goal is to avoid spinning and hammering memory, use pause?
>> 
>> Jess
> Yes, we try to avoid spinning and hammering memory. We will need
> testing to know if pause can help, too.
> 
> Thanks a lot!
> 
> Regards,
> Chao-ying