Re: [Qemu-devel] [PATCH v7 07/42] ram_debug_dump_bitmap: Dump a migration bitmap as text

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Juan Quintela (quintela@redhat.com) wrote:
> "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> > Useful for debugging the migration bitmap and other bitmaps
> > of the same format (including the sentmap in postcopy).
> >
> > The bitmap is printed to stderr.
> > Lines that are all the expected value are excluded so the output
> > can be quite compact for many bitmaps.
> >
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  include/migration/migration.h |  1 +
> >  migration/ram.c               | 38 ++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 39 insertions(+)
> >
> > diff --git a/include/migration/migration.h b/include/migration/migration.h
> > index 9387c8c..b3a7f75 100644
> > --- a/include/migration/migration.h
> > +++ b/include/migration/migration.h
> > @@ -144,6 +144,7 @@ uint64_t xbzrle_mig_pages_cache_miss(void);
> >  double xbzrle_mig_cache_miss_rate(void);
> >  
> >  void ram_handle_compressed(void *host, uint8_t ch, uint64_t size);
> > +void ram_debug_dump_bitmap(unsigned long *todump, bool expected);
> >  
> >  /**
> >   * @migrate_add_blocker - prevent migration from proceeding
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 57368e1..efc215a 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -1051,6 +1051,44 @@ static void reset_ram_globals(void)
> >  
> >  #define MAX_WAIT 50 /* ms, half buffered_file limit */
> >  
> > +/*
> > + * 'expected' is the value you expect the bitmap mostly to be full
> > + * of; it won't bother printing lines that are all this value.
> > + * If 'todump' is null the migration bitmap is dumped.
> > + */
> > +void ram_debug_dump_bitmap(unsigned long *todump, bool expected)
> > +{
> > +    int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS;
> > +
> > +    int64_t cur;
> > +    int64_t linelen = 128;
> > +    char linebuf[129];
> > +
> > +    if (!todump) {
> > +        todump = migration_bitmap;
> > +    }
> 
> Why?  Just alssert that todump!= NULL?

'migration_bitmap' is static to ram.c, so allowing NULL to get
you a dump of the migration_bitmap means that if you call this
dump routine from any error path you're debugging anywhere in qemu
then you can dump the migration bitmap.  e.g. I was adding calls
to this in migration.c and migration/postcopy-ram.c in error
paths I was trying to debug.

> > +    for (cur = 0; cur < ram_pages; cur += linelen) {
> > +        int64_t curb;
> > +        bool found = false;
> > +        /*
> > +         * Last line; catch the case where the line length
> > +         * is longer than remaining ram
> > +         */
> > +        if (cur + linelen > ram_pages) {
> > +            linelen = ram_pages - cur;
> > +        }
> > +        for (curb = 0; curb < linelen; curb++) {
> > +            bool thisbit = test_bit(cur + curb, todump);
> > +            linebuf[curb] = thisbit ? '1' : '.';
> 
> Put 1 and 0?  Why the dot?

It's easier to see an occasional '1' in a big field of .'s.

> > +            found = found || (thisbit != expected);
> > +        }
> > +        if (found) {
> > +            linebuf[curb] = '\0';
> > +            fprintf(stderr,  "0x%08" PRIx64 " : %s\n", cur, linebuf);
> > +        }
> > +    }
> > +}
> 
> 
> And once here, why are we doing it this way?  We have
> 
> find_first_bit(addr, nbits) and find_first_zero_bit(addr, nbits) and
> friends?
> 
> Doiwg the walk by hand looks weird, no?

Here's a compile-tested-only version using find_  - it's bigger, if you think
it's better I can use this instead:

void ram_debug_dump_bitmap(unsigned long *todump, bool expected)
{
    int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS;

    int64_t cur;
    int64_t linelen = 128;

    if (!todump) {
        todump = migration_bitmap;
    }

    for (cur = 0; cur < ram_pages; cur += linelen) {
        int64_t curb;
        unsigned long next_bit;

        /*
         * Last line; catch the case where the line length
         * is longer than remaining ram
         */
        if (cur + linelen > ram_pages) {
            linelen = ram_pages - cur;
        }
        if (expected) {
            next_bit = find_next_bit(todump, cur + linelen, cur);
        } else {
            next_bit = find_next_zero_bit(todump, cur + linelen, cur);
        }
        if (next_bit >= (cur + linelen)) {
            continue;
        }

        for (curb = 0; curb < linelen; curb++) {
            bool thisbit = test_bit(cur + curb, todump);
            fputc(thisbit ? '1' : '.', stderr);
        }
        fputc('\n', stderr);
    }
}

Dave


> 
> Later, Juan.
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK