On 17 December 2015 at 12:27, Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
> Am Donnerstag, den 17.12.2015, 12:20 +0100 schrieb David Jander:
>> Hi Lucas,
>>
>> Thanks for reacting.
>>
>> On Thu, 17 Dec 2015 12:03:10 +0100
>> Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
>>
>> > Am Donnerstag, den 17.12.2015, 11:28 +0100 schrieb David Jander:
>> > > Hi all,
>> > >
>> > > I was investigating the source of abnormal irq-latency spikes on an i.MX6
>> > > (ARM) board, and discovered this:
>> > >
>> > > # tracer: preemptirqsoff
>> > > #
>> > > # preemptirqsoff latency trace v1.1.5 on 4.4.0-rc4+
>> > > # --------------------------------------------------------------------
>> > > # latency: 2068 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:1)
>> > > # -----------------
>> > > # | task: mmcqd/0-92 (uid:0 nice:0 policy:0 rt_prio:0)
>> > > # -----------------
>> > > # => started at: _raw_spin_lock_irqsave
>> > > # => ended at: _raw_spin_unlock_irqrestore
>> > > #
>> > > #
>> > > # _------=> CPU#
>> > > # / _-----=> irqs-off
>> > > # | / _----=> need-resched
>> > > # || / _---=> hardirq/softirq
>> > > # ||| / _--=> preempt-depth
>> > > # |||| / delay
>> > > # cmd pid ||||| time | caller
>> > > # \ / ||||| \ | /
>> > > mmcqd/0-92 0d... 1us#: _raw_spin_lock_irqsave
>> > > mmcqd/0-92 0.n.1 2066us : _raw_spin_unlock_irqrestore
>> > > mmcqd/0-92 0.n.1 2070us+: trace_preempt_on
>> > > <-_raw_spin_unlock_irqrestore mmcqd/0-92 0.n.1 2107us : <stack trace>
>> > > => sdhci_runtime_resume_host
>> > > => __rpm_callback
>> > > => rpm_callback
>> > > => rpm_resume
>> > > => __pm_runtime_resume
>> > > => __mmc_claim_host
>> > > => mmc_blk_issue_rq
>> > > => mmc_queue_thread
>> > > => kthread
>> > > => ret_from_fork
>> > >
>> > > 2 ms with interrupts disabled!!! To much dismay, I later discovered that
>> > > this isn't even the worst case scenario. I also discovered that this has
>> > > been in the kernel for a long time without a fix (I have tested from 3.17
>> > > to 4.4-rc4). There has been an attempt by someone to address this back in
>> > > 2010, but apparently it never hit mainline.
>> > > Going through the code in sdhci.c, I found this troublesome code-path:
>> > >
>> > > sdhci_do_set_ios() {
>> > > spin_lock_irqsave(&host->lock, flags);
>> > > ...
>> > > sdhci_reinit() --> sdhci_init() --> sdhci_do_reset() -->
>> > > host->ops->reset() --> sdhci_reset()
>> > > ...
>> > > spin_unlock_irqrestore(&host->lock, flags);
>> > > }
>> > >
>> > > And in sdhci_reset(), which may be called with held spinlock:
>> > >
>> > > ...
>> > > /* Wait max 100 ms */
>> > > timeout = 100;
>> > >
>> > > /* hw clears the bit when it's done */
>> > > while (sdhci_readb(host, SDHCI_SOFTWARE_RESET) & mask) {
>> > > if (timeout == 0) {
>> > > pr_err("%s: Reset 0x%x never completed.\n",
>> > > mmc_hostname(host->mmc), (int)mask);
>> > > sdhci_dumpregs(host);
>> > > return;
>> > > }
>> > > timeout--;
>> > > mdelay(1);
>> > > }
>> > >
>> > > I am wondering: There either must be a reason this hasn't been fixed in
>> > > such a long time, or I am not understanding this correctly, so please
>> > > enlighten me. Before trying a cowboy attempt at "fixing" this, I'd really
>> > > like to know why am I seeing this?
>> > > I mean... how can such a problem get unnoticed and unfixed for so long?
>> > > Will an attempt at fixing this issue even be accepted?
>> > >
>> > I would like to see the sdhci spinlock killed and replaced by a mutex
>> > for exactly this reason.
>> >
>> > That said, your problem is card polling, when no card is present in the
>> > slot. This is most probably caused by CD gpios having the wrong
>> > polarity.
>>
>> ... or not having a CD pin at all.
>> I am using an embedded eMMC chip and a uSD card inserted into a slot. The card
>> is present and also detected as such. If I never access the card, I see no
>> spikes (filesystem is mounted but not accessed). If I try to read a file or
>> directory I get the above trace.
>> OTOH, if I disable PM functionality in the kernel, the spike is gone, and
>> worst-case latency is in the 300us range, so I don't think this is related to
>> card polling.
>>
> You may be right. If the SDHCI host gets runtime suspended it needs a
> reset after waking up, causing this latency.
>
> Killing the sdhci spinlock may not be straight forward, as a lot of code
> paths need to be audited for irq safety, but it's the only thing to fix
> this properly.
>
> Regards,
> Lucas
>
If/when you decide to fix this issue. Please keep in mind the following things.
- Try to convert the SDHCI into a pure library. No more quirks or callbacks.
- I assume we can simplify lots of code if we convert SDHCI into using
a threaded IRQ in favour of the tasklet.
Any patches that moves SDHCI into this direction will be greatly appreciated!
Kind regards
Uffe
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