Peter Zijlstra <[email protected]> writes: > On Fri, Jul 17, 2015 at 04:34:09PM +0300, Alexander Shishkin wrote: >> Currently, the PT driver zeroes out the status register every time before >> starting the event. However, all the writable bits are already taken care >> of in pt_handle_status() function, except the new PacketByteCnt field, >> which in new versions of PT contains the number of packet bytes written >> since the last sync (PSB) packet. Zeroing it out before enabling PT forces >> a sync packet to be written. This means that, with the existing code, a >> sync packet (PSB and PSBEND, 18 bytes in total) will be generated every >> time a PT event is scheduled in. >> >> To avoid these unnecessary syncs and save a WRMSR in the fast path, this >> patch adds a new attribute config bit "no_force_psb", which will disable >> this zeroing WRMSR. > > Why is this exposed?
The default behavior, which we have in the kernel now, is to always force the PSB, so if we change it, we need to make sure nobody is relying on it being the default behavior. Granted, for the hardware that's on the market right now it won't make a difference (it generates PSB+ at least every 256 bytes anyway), but in future if we change the default, newer hardware will produce different results with 4.2 than with the newer kernels and this would be a tad sloppy. The tools, however, can decide to set no_force_psb=1 when no_force_psb is available (sysfs caps) and otherwise the default behavior will be the same as no_force_psb==0. Makes sense? Regards, -- Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
