linux-kernel - Re: [RFC PATCH] perf cs-etm: Handle valid-but-zero timestamps

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Message-ID: <20210511080504.GC8273@leoy-ThinkPad-X240s>
Date:   Tue, 11 May 2021 16:05:04 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     coresight@...ts.linaro.org, mathieu.poirier@...aro.org,
        al.grant@....com, branislav.rankov@....com, denik@...omium.org,
        suzuki.poulose@....com, anshuman.khandual@....com,
        Mike Leach <mike.leach@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] perf cs-etm: Handle valid-but-zero timestamps

On Mon, May 10, 2021 at 01:39:04PM +0800, Leo Yan wrote:
> On Fri, May 07, 2021 at 01:02:35PM +0300, James Clark wrote:
> > 
> > 
> > On 07/05/2021 12:58, James Clark wrote:
> > > There is an intermittent issue on Trogdor devices that
> > > results in all Coresight timestamps having a value of zero.
> > 
> > I've attached a file here that has the issue. From the dump you 
> > can see the zero timestamps:
> > 
> >         Idx:69; ID:10;  I_TIMESTAMP : Timestamp.; Updated val = 0x0
> >         Idx:71; ID:10;  I_ATOM_F1 : Atom format 1.; E
> >         Idx:72; ID:10;  I_ADDR_S_IS0 : Address, Short, IS0.; Addr=0xFFFFFFE723C65824 ~[0x5824]
> > 
> > This doesn't have an impact on decoding as they end up being
> > decoded in file order as in with timeless mode.
> 
> Just remind, as Mike has mentioned that if the timestamp is zero, it
> means the hardware setting for timestamp is not enabled properly.  So
> for system wide or per CPU mode tracing, it's better to double check
> what's the reason the timestamp is not enabled properly.
> 
> IIUC, this patch breaks the existed rational in the code.  Let's think
> about there have 4 CPUs, every CPU has its own AUX trace buffer, and
> when decode the trace data, it will use 4 queues to track the packets
> and every queue has its timestamp.
> 
>   CPU0: cs_etm_queue -> ... -> packet_queue->timestamp
>   CPU1: cs_etm_queue -> ... -> packet_queue->timestamp
>   CPU2: cs_etm_queue -> ... -> packet_queue->timestamp
>   CPU3: cs_etm_queue -> ... -> packet_queue->timestamp
> 
> The issue is if all CPUs' timestamp are zero, it's impossible to find
> a way to synthesize samples in the right time order.

I saw Denis's replying for the hardware issue for timestamp, wander if
can add a new option "--force-aux-ts-zero" to override the hardware
timestamp issue.  Without the option "--force-aux-ts-zero", the
developers still have chance to observe the failure case caused by the
abnormal timestamps.

Thanks,
Leo