Message-ID: <20200409202714.GT20625@builder.lan>
Date:   Thu, 9 Apr 2020 13:27:14 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Rishabh Bhatnagar <rishabhb@...eaurora.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-remoteproc <linux-remoteproc@...r.kernel.org>,
        Ohad Ben-Cohen <ohad@...ery.com>, psodagud@...eaurora.org,
        tsoni@...eaurora.org, Siddharth Gupta <sidgup@...eaurora.org>
Subject: Re: [PATCH] remoteproc: core: Add a memory efficient coredump
 function

On Fri 03 Apr 13:53 PDT 2020, Mathieu Poirier wrote:

> On Thu, 2 Apr 2020 at 23:16, Bjorn Andersson <bjorn.andersson@...aro.org> wrote:
> >
> > On Thu 02 Apr 10:24 PDT 2020, Mathieu Poirier wrote:
> >
> > > On Wed, Apr 01, 2020 at 12:51:14PM -0700, Bjorn Andersson wrote:
> > > > On Fri 27 Mar 16:56 PDT 2020, Rishabh Bhatnagar wrote:
> > > >
> > > > > The current coredump implementation uses vmalloc area to copy
> > > > > all the segments. But this might put a lot of strain on low memory
> > > > > targets as the firmware size sometimes is in ten's of MBs.
> > > > > The situation becomes worse if there are multiple remote processors
> > > > > undergoing recovery at the same time.
> > > > > This patch directly copies the device memory to userspace buffer
> > > > > and avoids extra memory usage. This requires recovery to be halted
> > > > > until data is read by userspace and free function is called.
> > > > >
> > > > > Signed-off-by: Rishabh Bhatnagar <rishabhb@...eaurora.org>
> > > > > ---
> > > > >  drivers/remoteproc/remoteproc_core.c | 107 +++++++++++++++++++++++++++++------
> > > > >  include/linux/remoteproc.h           |   4 ++
> > > > >  2 files changed, 94 insertions(+), 17 deletions(-)
> > > > >
> > > > > diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
> > > > > index 097f33e..2d881e5 100644
> > > > > --- a/drivers/remoteproc/remoteproc_core.c
> > > > > +++ b/drivers/remoteproc/remoteproc_core.c
> > > > > @@ -1516,6 +1516,86 @@ int rproc_coredump_add_segment(struct rproc *rproc, dma_addr_t da, size_t size)
> > > > >  }
> > > > >  EXPORT_SYMBOL(rproc_coredump_add_segment);
> > > > >
> > > > > +
> > > > > +void rproc_free_dump(void *data)
> > > >
> > > > static
> > > >
> > > > > +{
> > > > > + struct rproc *rproc = data;
> > > > > +
> > > > > + dev_info(&rproc->dev, "Userspace done reading rproc dump\n");
> > > >
> > > > Please drop the info prints throughout.
> > > >
> > > > > + complete(&rproc->dump_done);
> > > > > +}
> > > > > +
> > > > > +static unsigned long get_offset(loff_t user_offset, struct list_head *segments,
> > > > > +                         unsigned long *data_left)
> > > >
> > > > Please rename this rproc_coredump_resolve_segment(), or something along
> > > > those lines.
> > > >
> > > > > +{
> > > > > + struct rproc_dump_segment *segment;
> > > > > +
> > > > > + list_for_each_entry(segment, segments, node) {
> > > > > +         if (user_offset >= segment->size)
> > > > > +                 user_offset -= segment->size;
> > > > > +         else
> > > > > +                 break;
> > > > > + }
> > > > > +
> > > > > + if (&segment->node == segments) {
> > > > > +         *data_left = 0;
> > > > > +         return 0;
> > > > > + }
> > > > > +
> > > > > + *data_left = segment->size - user_offset;
> > > > > +
> > > > > + return segment->da + user_offset;
> > > > > +}
> > > > > +
> > > > > +static ssize_t rproc_read_dump(char *buffer, loff_t offset, size_t count,
> > > > > +                         void *data, size_t elfcorelen)
> > > > > +{
> > > > > + void *device_mem = NULL;
> > > > > + unsigned long data_left = 0;
> > > > > + unsigned long bytes_left = count;
> > > > > + unsigned long addr = 0;
> > > > > + size_t copy_size = 0;
> > > > > + struct rproc *rproc = data;
> > > > > +
> > > > > + if (offset < elfcorelen) {
> > > > > +         copy_size = elfcorelen - offset;
> > > > > +         copy_size = min(copy_size, bytes_left);
> > > > > +
> > > > > +         memcpy(buffer, rproc->elfcore + offset, copy_size);
> > > > > +         offset += copy_size;
> > > > > +         bytes_left -= copy_size;
> > > > > +         buffer += copy_size;
> > > > > + }
> > > > > +
> > > > > + while (bytes_left) {
> > > > > +         addr = get_offset(offset - elfcorelen, &rproc->dump_segments,
> > > > > +                         &data_left);
> > > > > + /* EOF check */
> > > >
> > > > Indentation, and "if no data left" does indicate that this is the end of
> > > > the loop already.
> > > >
> > > > > +         if (data_left == 0) {
> > > > > +                 pr_info("Ramdump complete. %lld bytes read.", offset);
> > > > > +                 return 0;
> > > >
> > > > You might have copied data to the buffer, so returning 0 here doesn't
> > > > seem right. Presumably instead you should break and return offset -
> > > > original offset or something like that.
> > > >
> > > > > +         }
> > > > > +
> > > > > +         copy_size = min_t(size_t, bytes_left, data_left);
> > > > > +
> > > > > +         device_mem = rproc->ops->da_to_va(rproc, addr, copy_size);
> > > > > +         if (!device_mem) {
> > > > > +                 pr_err("Unable to ioremap: addr %lx, size %zd\n",
> > > > > +                          addr, copy_size);
> > > > > +                 return -ENOMEM;
> > > > > +         }
> > > > > +         memcpy(buffer, device_mem, copy_size);
> > > > > +
> > > > > +         offset += copy_size;
> > > > > +         buffer += copy_size;
> > > > > +         bytes_left -= copy_size;
> > > > > +         dev_dbg(&rproc->dev, "Copied %d bytes to userspace\n",
> > > > > +                 copy_size);
> > > > > + }
> > > > > +
> > > > > + return count;
> > > >
> > > > This should be the number of bytes actually returned, so if count is
> > > > larger than the sum of the segment sizes this will be wrong.
> > > >
> > > > > +}
> > > > > +
> > > > >  /**
> > > > >   * rproc_coredump_add_custom_segment() - add custom coredump segment
> > > > >   * @rproc:       handle of a remote processor
> > > > > @@ -1566,27 +1646,27 @@ static void rproc_coredump(struct rproc *rproc)
> > > > >   struct rproc_dump_segment *segment;
> > > > >   struct elf32_phdr *phdr;
> > > > >   struct elf32_hdr *ehdr;
> > > > > - size_t data_size;
> > > > > + size_t header_size;
> > > > >   size_t offset;
> > > > >   void *data;
> > > > > - void *ptr;
> > > > >   int phnum = 0;
> > > > >
> > > > >   if (list_empty(&rproc->dump_segments))
> > > > >           return;
> > > > >
> > > > > - data_size = sizeof(*ehdr);
> > > > > + header_size = sizeof(*ehdr);
> > > > >   list_for_each_entry(segment, &rproc->dump_segments, node) {
> > > > > -         data_size += sizeof(*phdr) + segment->size;
> > > > > +         header_size += sizeof(*phdr);
> > > > >
> > > > >           phnum++;
> > > > >   }
> > > > >
> > > > > - data = vmalloc(data_size);
> > > > > + data = vmalloc(header_size);
> > > > >   if (!data)
> > > > >           return;
> > > > >
> > > > >   ehdr = data;
> > > > > + rproc->elfcore = data;
> > > >
> > > > Rather than using a rproc-global variable I would prefer that you create
> > > > a new rproc_coredump_state struct that carries the header pointer and
> > > > the information needed by the read & free functions.
> > > >
> > > > >
> > > > >   memset(ehdr, 0, sizeof(*ehdr));
> > > > >   memcpy(ehdr->e_ident, ELFMAG, SELFMAG);
> > > > > @@ -1618,23 +1698,14 @@ static void rproc_coredump(struct rproc *rproc)
> > > > >
> > > > >           if (segment->dump) {
> > > > >                   segment->dump(rproc, segment, data + offset);
> > >
> > > I'm not exactly sure why custom segments can be copied to the elf image but not
> > > generic ones... And as far as I can tell accessing "data + offset" will blow up
> > > because only the memory for the program headers has been allocated, not for the
> > > program segments.
> > >
> >
> > Thanks, I missed that, but you're correct.
> >
> > >
> > > > > -         } else {
> > > > > -                 ptr = rproc_da_to_va(rproc, segment->da, segment->size);
> > > > > -                 if (!ptr) {
> > > > > -                         dev_err(&rproc->dev,
> > > > > -                                 "invalid coredump segment (%pad, %zu)\n",
> > > > > -                                 &segment->da, segment->size);
> > > > > -                         memset(data + offset, 0xff, segment->size);
> > > > > -                 } else {
> > > > > -                         memcpy(data + offset, ptr, segment->size);
> > > > > -                 }
> > > > > -         }
> > > > >
> > > > >           offset += phdr->p_filesz;
> > > > >           phdr++;
> > > > >   }
> > > > > + dev_coredumpm(&rproc->dev, NULL, rproc, header_size, GFP_KERNEL,
> > > > > +                 rproc_read_dump, rproc_free_dump);
> > > > >
> > > > > - dev_coredumpv(&rproc->dev, data, data_size, GFP_KERNEL);
> > > > > + wait_for_completion(&rproc->dump_done);
> > > >
> > > > This will mean that recovery handling will break on installations that
> > > > doesn't have your ramdump collector - as it will just sit here forever
> > > > (5 minutes) waiting for userspace to do its job.
> > >
> > > Right, that problem also came to mind.
> > >
> > > >
> > > > I think we need to device a new sysfs attribute, through which you can
> > > > enable the "inline" coredump mechanism. That way recovery would work for
> > > > all systems and in your specific case you could reconfigure it - perhaps
> > > > once the ramdump collector starts.
> > >
> > > Another option is to make rproc_coredump() customizable, as with all the other
> > > functions in remoteproc_internal.h.  That way the current rproc_coredump() is
> > > kept intact and we don't need a new sysfs entry.
> > >
> >
> > Rishabh suggested this in a discussion we had earlier this week as well,
> > but we still have the problem that the same platform driver will need to
> > support both modes, depending on which user space is running. So even if
> > we push this out to the platform driver we still need some mechanism
> > for userspace to enable the "inline" mode.
> 
> So is this something that needs to be done on the fly in response to
> some system event?  Any possibility to use the DT?
> 

Designing this as a dynamic property would mean that the kernel doesn't
have to be recompiled between different variants of the same software
solution for a piece of hardware.

Putting a flag in DT would mean that you need to flash different DT
depending on what apps your running in userspace.

> We are currently discussing the addition of a character driver [1]...
> The file_operations could be platform specific so any scenario can be
> implemented, whether it is switching on/off a remote processor in the
> open/release() callback or setting the behavior of the coredump
> functionality in an ioctl().

The main benefit of tying this to the character device would be that the
behavior could be reverted on release(). But this would imply that the
application starting and stopping the remoteproc is also the one
collecting ramdumps and it would also imply that there exists such an
application (e.g. this functionality is still desirable for auto_booted
remoteprocs).

Finally I think it's likely that the existing tools for collecting
devcoredump artifacts are expected to just continue to work after this
change - in both modes.



On the functionality Rishabh proposes, it would be very interesting to
hear from others on their usage and need for coredumps.

E.g. are Qualcomm really the only ones that has issues with
vmalloc(sizeof(firmware)) failing and preventing post mortem debugging
in low-memory scenarios? Or does others simply not care to debug
remoteproc firmware in these cases? Is debugging only done using JTAG?

> I think there is value in exploring different opportunities so that we
> keep the core as clean and simple as possible.
> 

I agree, but hadn't considered this fully. In particular with the
changes I'm asking Rishabh to make we have a few screen fulls of code
involved in the coredump handling. So I think it would be beneficial to
move this into a remoteproc_coredump.c.

Thanks,
Bjorn

> Thanks,
> Mathieu
> 
> [1]. https://patchwork.kernel.org/project/linux-remoteproc/list/?series=264603
> 
> >
> > Regards,
> > Bjorn

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