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January 6, 2020 01:07 am

Linus Torvalds Calls Blogger's Linux Scheduler Tests 'Pure Garbage'

On Wednesday Phoronix cited a blog post by C++ game developer Malte Skarupke claiming his spinlocks experiments had discovered the Linux kernel had a scheduler issue affecting developers bringing games to Linux for Google Stadia. Linus Torvalds has now responded:The whole post seems to be just wrong, and is measuring something completely different than what the author thinks and claims it is measuring. First off, spinlocks can only be used if you actually know you're not being scheduled while using them. But the blog post author seems to be implementing his own spinlocks in user space with no regard for whether the lock user might be scheduled or not. And the code used for the claimed "lock not held" timing is complete garbage. It basically reads the time before releasing the lock, and then it reads it after acquiring the lock again, and claims that the time difference is the time when no lock was held. Which is just inane and pointless and completely wrong... [T]he code in question is pure garbage. You can't do spinlocks like that. Or rather, you very much can do them like that, and when you do that you are measuring random latencies and getting nonsensical values, because what you are measuring is "I have a lot of busywork, where all the processes are CPU-bound, and I'm measuring random points of how long the scheduler kept the process in place". And then you write a blog-post blamings others, not understanding that it's your incorrect code that is garbage, and is giving random garbage values... You might even see issues like "when I run this as a foreground UI process, I get different numbers than when I run it in the background as a batch process". Cool interesting numbers, aren't they? No, they aren't cool and interesting at all, you've just created a particularly bad random number generator... [Y]ou should never ever think that you're clever enough to write your own locking routines.. Because the likelihood is that you aren't (and by that "you" I very much include myself -- we've tweaked all the in-kernel locking over decades, and gone through the simple test-and-set to ticket locks to cacheline-efficient queuing locks, and even people who know what they are doing tend to get it wrong several times). There's a reason why you can find decades of academic papers on locking. Really. It's hard. "It really means a lot to me that Linus responded," the blogger wrote later, "even if the response is negative." They replied to Torvalds' 1,500-word post on the same mailing list -- and this time received a 1900-word response arguing "you did locking fundamentally wrong..."The fact is, doing your own locking is hard. You need to really understand the issues, and you need to not over-simplify your model of the world to the point where it isn't actually describing reality any more... Dealing with reality is hard. It sometimes means that you need to make your mental model for how locking needs to work a lot more complicated...

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