Re: [PATCH/RFC] perf buildid: Cache kernel DSO created when reading buildid header table

[Namhyung Kim]

Hi Arnaldo,

On Thu, Sep 24, 2015 at 10:42:28AM -0300, Arnaldo Carvalho de Melo wrote:
> Namhyung,
> 
> 	Can you take a look and perhaps give me your Acked-by?

The dso->kernel and event->cpumode is always confusing for modules..
In this case, it seems that mmap events set kernel cpumode but
build-id events don't.  So kernel cpumode in a bulid-id event
indicates that it is a kernel (vmlinux) dso, right?

I'll test this tomorrow..

Thanks,
Namhyung


> 
> From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 
> No need to traverse it all again using the filename extension to
> disambiguate kernel from kernel modules, just cache it when
> reading the buildid table.
> 
> This also fixes a refcount bug in the error path, i.e. in the error path
> for the __machine__create_kernel_maps() call we do a dso__put(), which
> is ok and expected if that dso came from machine__findnew_dso(), but
> wasn't when we found it by traversing the machine dso list without
> grabbing a dso refcount (dso__get()) before dropping the list lock.
> 
> Fixes: b837a8bdc489 ("perf tools: Fix build-id matching on vmlinux")
> Link: http://lkml.kernel.org/n/tip-48fh3cf6pvs4zs2fj4nhc4b5@xxxxxxxxxxxxxx
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>  tools/perf/util/header.c  |  7 +++++++
>  tools/perf/util/machine.c | 34 +++-------------------------------
>  tools/perf/util/machine.h |  1 +
>  3 files changed, 11 insertions(+), 31 deletions(-)
> 
> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> index 43838003c1a1..6bc92ae1e99a 100644
> --- a/tools/perf/util/header.c
> +++ b/tools/perf/util/header.c
> @@ -1260,6 +1260,13 @@ static int __event_process_build_id(struct build_id_event *bev,
>  
>  		if (!is_kernel_module(filename, cpumode))
>  			dso->kernel = dso_type;
> +		/*
> +		 * We don't need to grab a reference as long as
> +		 * the kernel dso is in the machine dso list.
> +		 */
> +		if (cpumode == PERF_RECORD_MISC_KERNEL ||
> +		    cpumode == PERF_RECORD_MISC_GUEST_KERNEL)
> +			machine->kernel_dso = dso;
>  
>  		build_id__sprintf(dso->build_id, sizeof(dso->build_id),
>  				  sbuild_id);
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index d07a38678e14..f6e689b2c83c 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -62,6 +62,7 @@ int machine__init(struct machine *machine, const char *root_dir, pid_t pid)
>  	}
>  
>  	machine->current_tid = NULL;
> +	machine->kernel_dso = NULL;
>  
>  	return 0;
>  }
> @@ -1180,39 +1181,10 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
>  		 * Should be there already, from the build-id table in
>  		 * the header.
>  		 */
> -		struct dso *kernel = NULL;
> -		struct dso *dso;
> +		struct dso *kernel;
>  
>  		pthread_rwlock_rdlock(&machine->dsos.lock);
> -
> -		list_for_each_entry(dso, &machine->dsos.head, node) {
> -
> -			/*
> -			 * The cpumode passed to is_kernel_module is not the
> -			 * cpumode of *this* event. If we insist on passing
> -			 * correct cpumode to is_kernel_module, we should
> -			 * record the cpumode when we adding this dso to the
> -			 * linked list.
> -			 *
> -			 * However we don't really need passing correct
> -			 * cpumode.  We know the correct cpumode must be kernel
> -			 * mode (if not, we should not link it onto kernel_dsos
> -			 * list).
> -			 *
> -			 * Therefore, we pass PERF_RECORD_MISC_CPUMODE_UNKNOWN.
> -			 * is_kernel_module() treats it as a kernel cpumode.
> -			 */
> -
> -			if (!dso->kernel ||
> -			    is_kernel_module(dso->long_name,
> -					     PERF_RECORD_MISC_CPUMODE_UNKNOWN))
> -				continue;
> -
> -
> -			kernel = dso;
> -			break;
> -		}
> -
> +		kernel = dso__get(machine->kernel_dso);
>  		pthread_rwlock_unlock(&machine->dsos.lock);
>  
>  		if (kernel == NULL)
> diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
> index 9dfc4281f940..d7b044a1046f 100644
> --- a/tools/perf/util/machine.h
> +++ b/tools/perf/util/machine.h
> @@ -38,6 +38,7 @@ struct machine {
>  	struct dsos	  dsos;
>  	struct map_groups kmaps;
>  	struct map	  *vmlinux_maps[MAP__NR_TYPES];
> +	struct dso	  *kernel_dso;
>  	u64		  kernel_start;
>  	symbol_filter_t	  symbol_filter;
>  	pid_t		  *current_tid;
> -- 
> 2.1.0
> 
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