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

[Juan Quintela]

"Dr. David Alan Gilbert" <address@hidden> wrote:
> * Juan Quintela (address@hidden) wrote:
>> "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
>> > From: "Dr. David Alan Gilbert" <address@hidden>
>> >
>> > 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 <address@hidden>
>> > ---
>> >  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.

ok.

>
>> > +    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.

ok.

>
>> > +            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

Reviewed-by: Juan Quintela <address@hidden>


>
>
>> 
>> Later, Juan.
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK