On Thu, 19 Jun 2014, Tejun Heo wrote: > On Thu, Jun 19, 2014 at 03:42:07PM -0500, Christoph Lameter wrote: > > In that case special care needs to be taken to get this right. True. > > > > I typically avoid these scenarios by sending an IPI with a pointer to the > > data structure. The modification is done by the cpu for which the per cpu > > data is local. > > > > Maybe rewrite the code to avoid writing to other processors percpu data > > would be the right approach? > > It depends on the specific use case but in general no. IPIs would be > far more expensive than making use of proper barriers in vast majority > of cases especially when the "hot" side is data dependency barrier, > IOW, nothing. Also, we are talking about extremely low frequency > events like init and recycling after reinit. Regular per-cpu > operation isn't really the subject here.
The aim of having percpu data is to have the ability for a processor to access memory dedicated to that processor in the fastest way possible by avoiding synchronization. You are beginning to add synchronization elements into the accesses of a processor to memory dedicated to its sole use. Remote write events are contrary to that design and are exceedingly rare. An IPI is justifiable for such a rare event. At least in my use cases I have always found that to be sufficient. Well, I designed the data structures in a way that made this possible because of the design criteria that did not allow me remote write access to other processors per cpu data. -- 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/
