On Mon, Mar 23, 2020 at 8:13 PM Mattias Rönnblom
<mattias.ronnb...@ericsson.com> wrote:
>
> On 2020-03-23 14:37, Jerin Jacob wrote:
> >>> + }
> >>> +
> >>> + /* Initialize the trace point */
> >>> + if (rte_strscpy(tp->name, name, TRACE_POINT_NAME_SIZE) < 0) {
> >>> + trace_err("name is too long");
> >>> + rte_errno = E2BIG;
> >>> + goto free;
> >>> + }
> >>> +
> >>> + /* Copy the field data for future use */
> >>> + if (rte_strscpy(tp->ctf_field, field, TRACE_CTF_FIELD_SIZE) < 0) {
> >>> + trace_err("CTF field size is too long");
> >>> + rte_errno = E2BIG;
> >>> + goto free;
> >>> + }
> >>> +
> >>> + /* Clear field memory for the next event */
> >>> + memset(field, 0, TRACE_CTF_FIELD_SIZE);
> >>> +
> >>> + /* Form the trace handle */
> >>> + *handle = sz;
> >>> + *handle |= trace.nb_trace_points << __RTE_TRACE_FIELD_ID_SHIFT;
> >>> + *handle |= (uint64_t)level << __RTE_TRACE_FIELD_LEVEL_SHIFT;
> >> If *handle would be a struct, you could use a bitfield instead, and much
> >> simplify this code.
> > I thought that initially, Two reasons why I did not do that
> > 1) The flags have been used in fastpath, I prefer to work with flags
> > in fastpath so that
> Is it really that obvious that flags are faster than bitfield
> operations? I think most modern architectures have machine instructions
> for bitfield manipulation.
Add x86 maintainers.
There were comments in ml about bitfield inefficiency usage with x86.
http://patches.dpdk.org/patch/16482/
Search for: Bitfileds are efficient on Octeon. What's about other CPUs
you have in
mind? x86 is not as efficient.
Thoughts from x86 folks.
> > there is no performance impact using bitfields from the compiler _if any_.
> > 2) In some of the places, I can simply operate on APIs like
> > __atomic_and_fetch() with flags.
>
> I think you may still use such atomic operations. Just convert the
> struct to a uint64_t, which will essentially be a no-operation, and fire
> away.
Not sure, We think about the atomic "and" and fetch here.
That memcpy may translate additional load/store based on the compiler
optimization level.(say compiled with -O0)
>
>
> static uint64_t
>
> __rte_trace_raw(struct trace *t)
>
> {
>
> uint64_t raw;
>
> memcpy(&raw, t, sizeof(struct trace));
>
> return raw;
>
> }
>
>
