Hi Leon, On Sat, 7 Nov 2009 20:01:59 +0100, Leon Woestenberg wrote: > during testing the Linux PREEMP_RT work (step-by-step being merged > with mainline) together with I2C functionality I hit the fact that the > I2C subsystem uses yield() in some of the non-happy code paths (mostly > during chip / address probing etc). The I2C subsystem itself doesn't; individual I2C bus drivers do. One of them is i2c-algo-bit, which is a helper module widely used by other I2C bus drivers. yield() is only used once in i2c-algo-bit, when the slave device we are talking to doesn't ack its address at first try (and adapter->retries is set to non-zero.) > My (embedded) system was running a low-priority real-time work on the > generic workqueue which tried to blink a LED using an I2C I/O > multiplexer when I hit the BUG where this real-time task ran into the > yield() of try_address(). One thing I do not understand: if yield() is a bug to RT kernels, then we would have to remove them all? But so far, yield() still exists in the kernel tree, and it serves a purpose. Are you going to ask all developers to remove all occurrences of yield() in their code? Doesn't sound terribly realistic. > Grepping through the I2C subsystem code there where more yield() > sprinkled in, without it being clear to me why they are there. The only occurrence I found is in driver i2c-bfin-twi, where it is used to wait until the bus is ready. The use of yield() make the busy-waiting less aggressive. Alternatives are sleeping (which I presume RT wouldn't like either) or pure busy-waiting (which doesn't sound terribly appealing, right?) > Can those yield()s please be removed, and if they are needed for some > reason (??) be replaced with something equivalent? I don't think yield() is ever "needed". It is there when developers try to be fair to other running threads. I can't think of cases where anything will break when removing them, but system latency might suffer. Isn't it a little odd that in the name of RT, you're asking for people to remove a mechanism which was introduced to lower system latency? I think this all needs to be discussed at a higher level. Namely, RT people need to discuss how yield() should behave with regards to RT threads. Maybe it should simply become a no-op for these threads? (Disclaimer if it wasn't obvious yet: I don't know much about RT.) -- Jean Delvare -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
