Date:   Thu, 19 Apr 2018 09:40:30 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Indranil Choudhury <indranil@...lsio.com>,
        Nirranjan Kirubaharan <nirranjan@...lsio.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        Ganesh GR <ganeshgr@...lsio.com>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>
Subject: Re: [PATCH net-next v4 0/3] kernel: add support to collect hardware
 logs in crash recovery kernel

On 04/18/18 at 06:01pm, Rahul Lakkireddy wrote:
> On Wednesday, April 04/18/18, 2018 at 11:45:46 +0530, Dave Young wrote:
> > Hi Rahul,
> > On 04/17/18 at 01:14pm, Rahul Lakkireddy wrote:
> > > On production servers running variety of workloads over time, kernel
> > > panic can happen sporadically after days or even months. It is
> > > important to collect as much debug logs as possible to root cause
> > > and fix the problem, that may not be easy to reproduce. Snapshot of
> > > underlying hardware/firmware state (like register dump, firmware
> > > logs, adapter memory, etc.), at the time of kernel panic will be very
> > > helpful while debugging the culprit device driver.
> > > 
> > > This series of patches add new generic framework that enable device
> > > drivers to collect device specific snapshot of the hardware/firmware
> > > state of the underlying device in the crash recovery kernel. In crash
> > > recovery kernel, the collected logs are added as elf notes to
> > > /proc/vmcore, which is copied by user space scripts for post-analysis.
> > > 
> > > The sequence of actions done by device drivers to append their device
> > > specific hardware/firmware logs to /proc/vmcore are as follows:
> > > 
> > > 1. During probe (before hardware is initialized), device drivers
> > > register to the vmcore module (via vmcore_add_device_dump()), with
> > > callback function, along with buffer size and log name needed for
> > > firmware/hardware log collection.
> > 
> > I assumed the elf notes info should be prepared while kexec_[file_]load
> > phase. But I did not read the old comment, not sure if it has been discussed
> > or not.
> > 
> 
> We must not collect dumps in crashing kernel. Adding more things in
> crash dump path risks not collecting vmcore at all. Eric had
> discussed this in more detail at:
> 
> https://lkml.org/lkml/2018/3/24/319
> 
> We are safe to collect dumps in the second kernel. Each device dump
> will be exported as an elf note in /proc/vmcore.

I understand that we should avoid adding anything in crash path.  And I also
agree to collect device dump in second kernel.  I just assumed device
dump use some memory area to store the debug info and the memory
is persistent so that this can be done in 2 steps, first register the
address in elf header in kexec_load, then collect the dump in 2nd
kernel.  But it seems the driver is doing some other logic to collect
the info instead of just that simple like I thought. 

> 
> > If do this in 2nd kernel a question is driver can be loaded later than vmcore init.
> 
> Yes, drivers will add their device dumps after vmcore init.
> 
> > How to guarantee the function works if vmcore reading happens before
> > the driver is loaded?
> > 
> > Also it is possible that kdump initramfs does not contains the driver
> > module.
> > 
> > Am I missing something?
> > 
> 
> Yes, driver must be in initramfs if it wants to collect and add device
> dump to /proc/vmcore in second kernel.

In RH/Fedora kdump scripts we only add the things are required to
bring up the dump target, so that we can use as less memory as we can.

For example, if a net driver panicked, and the dump target is rootfs
which is a scsi disk, then no network related stuff will be added in
initramfs.

In this case the device dump info will be not collected..
> 
> > > 
> > > 2. vmcore module allocates the buffer with requested size. It adds
> > > an elf note and invokes the device driver's registered callback
> > > function.
> > > 
> > > 3. Device driver collects all hardware/firmware logs into the buffer
> > > and returns control back to vmcore module.
> > > 
> > > The device specific hardware/firmware logs can be seen as elf notes:
> > > 
> > > # readelf -n /proc/vmcore
> > > 
> > > Displaying notes found at file offset 0x00001000 with length 0x04003288:
> > >   Owner                 Data size	Description
> > >   VMCOREDD_cxgb4_0000:02:00.4 0x02000fd8	Unknown note type: (0x00000700)
> > >   VMCOREDD_cxgb4_0000:04:00.4 0x02000fd8	Unknown note type: (0x00000700)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   CORE                 0x00000150	NT_PRSTATUS (prstatus structure)
> > >   VMCOREINFO           0x0000074f	Unknown note type: (0x00000000)
> > > 
> > > Patch 1 adds API to vmcore module to allow drivers to register callback
> > > to collect the device specific hardware/firmware logs.  The logs will
> > > be added to /proc/vmcore as elf notes.
> > > 
> > > Patch 2 updates read and mmap logic to append device specific hardware/
> > > firmware logs as elf notes.
> > > 
> > > Patch 3 shows a cxgb4 driver example using the API to collect
> > > hardware/firmware logs in crash recovery kernel, before hardware is
> > > initialized.
> > > 
> > > Thanks,
> > > Rahul
> > > 
> > > RFC v1: https://lkml.org/lkml/2018/3/2/542
> > > RFC v2: https://lkml.org/lkml/2018/3/16/326
> > > 
> [...]
> 
> Thanks,
> Rahul

Thanks
Dave

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