NAME¶

tracefs_instance_get_subbuf_size, tracefs_instance_set_subbuf_size - Helper functions for working with ring buffer sub buffers.

SYNOPSIS¶

#include <tracefs.h>
size_t tracefs_instance_get_subbuf_size(struct tracefs_instance *instance);
int tracefs_instance_set_subbuf_size(struct tracefs_instance *instance, size_t size);

DESCRIPTION¶

Helper functions for working with the sub-buffers of the tracing ring buffer. The tracing ring buffer is broken up into sub-buffers. An event can not be bigger than the data section of the sub-buffer (see tep_get_sub_buffer_data_size(3)). By default, the ring buffer uses the architectures page_size as the default size of the sub-buffer, but this can be limiting if there is a need for large events, for example, the application wants to write large strings into the trace_marker file.

The tracefs_instance_get_subbuf_size() returns the current size in kilobytes fo the ring buffer sub-buffers.

The tracefs_instance_set_subbuf_size() will write the size in kilobytes of what the new sub-buffer size should be. Note, that this is only a hint to what the minimum sub-buffer size should be. It also does not take into account the meta-data that is used by the sub-buffer, so the size written should be no less than 16 bytes more than the maximum event size that will be used. The kernel will likely make the sub-buffer size larger than specified, as it may need to align the size for implementation purposes.

RETURN VALUE¶

The tracefs_instance_get_subbuf_size() returns the size of the current sub-buffer for the given instance ring buffer or -1 on error.

The tracefs_instance_set_subbuf_size() will return 0 if it successfully set the instance ring buffer sub-buffer size in kilobytes, or -1 on error.

EXAMPLE¶

#include <stdlib.h>
#include <tracefs.h>
#include <errno.h>
int main(int argc, char **argv)
{


        struct tep_handle *tep;


        ssize_t save_subsize;


        ssize_t subsize;


        char *trace;


        char buf[3000];


        int meta_size;


        int ret;


        int i;


        tep = tep_alloc();


        ret = tracefs_load_headers(NULL, tep);


        tep_free(tep);


        if (ret < 0) {


                perror("reading headers");


                exit(-1);


        }


        meta_size = tep_get_sub_buffer_size(tep) - tep_get_sub_buffer_data_size(tep);


        save_subsize = tracefs_instance_get_subbuf_size(NULL);


        if (save_subsize < 0) {


                printf("Changing sub-buffer size not available\n");


                exit(-1);


        }


        subsize = save_subsize * 1024;


        /* Have at least 4 writes fit on a sub-buffer */


        if (subsize - meta_size < sizeof(buf) *4 ) {


                subsize = ((sizeof(buf) * 4 + meta_size) + 1023) / 1024;


                tracefs_instance_set_subbuf_size(NULL, subsize);


        }


        for (i = 0; i < sizeof(buf) - 1; i++) {


                buf[i] = '0' + i % 10;


        }


        buf[i] = '\0';


        tracefs_instance_clear(NULL);


        for (i = 0; i < 4; i++) {


                ret = tracefs_printf(NULL, "%s\n", buf);


                if (ret < 0)


                        perror("write");


        }


        trace = tracefs_instance_file_read(NULL, "trace", NULL);


        printf("%s\n", trace);


        free(trace);


        printf("Buffer size was: %zd * 1024\n",


               tracefs_instance_get_subbuf_size(NULL));


        tracefs_instance_set_subbuf_size(NULL, save_subsize);
}

FILES¶

tracefs.h


        Header file to include in order to have access to the library APIs.
-ltracefs


        Linker switch to add when building a program that uses the library.

SEE ALSO¶

libtracefs(3), libtraceevent(3), trace-cmd(1)

AUTHOR¶

Steven Rostedt <rostedt@goodmis.org[1]>
Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>

REPORTING BUGS¶

Report bugs to <linux-trace-devel@vger.kernel.org[3]>

LICENSE¶

libtracefs is Free Software licensed under the GNU LGPL 2.1

RESOURCES¶

https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/

COPYING¶

Copyright (C) 2020 VMware, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).

NOTES¶

1.

rostedt@goodmis.org

2.

tz.stoyanov@gmail.com

3.

linux-trace-devel@vger.kernel.org