From 1902d94c9231f5a31466e73c1de08ad282674c3c Mon Sep 17 00:00:00 2001 From: Kent Overstreet Date: Mon, 29 Apr 2024 03:15:21 -0400 Subject: bcachefs: Developer guide Signed-off-by: Kent Overstreet --- .../filesystems/bcachefs/developer-guide.rst | 177 +++++++++++++++++++++ Documentation/filesystems/bcachefs/index.rst | 1 + 2 files changed, 178 insertions(+) create mode 100644 Documentation/filesystems/bcachefs/developer-guide.rst diff --git a/Documentation/filesystems/bcachefs/developer-guide.rst b/Documentation/filesystems/bcachefs/developer-guide.rst new file mode 100644 index 000000000000..47e851c3fba5 --- /dev/null +++ b/Documentation/filesystems/bcachefs/developer-guide.rst @@ -0,0 +1,177 @@ +Good development is like gardening, and codebases are our gardens. Tend to them +every day; look for little things that are out of place or in need of tidying. +A little weeding here and there goes a long way; don't wait until things have +spiraled out of control. + +Things don't always have to be perfect - nitpicking often does more harm than +good. But appreciate beauty when you see it - and let people know. + +The code that you are afraid to touch is the code most in need of refactoring. + +A little organizing here and there goes a long way. + +Put real thought into how you organize things. + +Good code is readable code, where the structure is simple and leaves nowhere +for bugs to hide. + +Assertions are one of our most important tools for writing reliable code. If in +the course of writing a patchset you encounter a condition that shouldn't +happen (and will have unpredictable or undefined behaviour if it does), or +you're not sure if it can happen and not sure how to handle it yet - make it a +BUG_ON(). Don't leave undefined or unspecified behavior lurking in the codebase. + +By the time you finish the patchset, you should understand better which +assertions need to be handled and turned into checks with error paths, and +which should be logically impossible. Leave the BUG_ON()s in for the ones which +are logically impossible. (Or, make them debug mode assertions if they're +expensive - but don't turn everything into a debug mode assertion, so that +we're not stuck debugging undefined behaviour should it turn out that you were +wrong). + +Assertions are documentation that can't go out of date. Good assertions are +wonderful. + +Good assertions drastically and dramatically reduce the amount of testing +required to shake out bugs. + +Good assertions are based on state, not logic. To write good assertions, you +have to think about what the invariants on your state are. + +Good invariants and assertions will hold everywhere in your codebase. This +means that you can run them in only a few places in the checked in version, but +should you need to debug something that caused the assertion to fail, you can +quickly shotgun them everywhere to find the codepath that broke the invariant. + +A good assertion checks something that the compiler could check for us, and +elide - if we were working in a language with embedded correctness proofs that +the compiler could check. This is something that exists today, but it'll likely +still be a few decades before it comes to systems programming languages. But we +can still incorporate that kind of thinking into our code and document the +invariants with runtime checks - much like the way people working in +dynamically typed languages may add type annotations, gradually making their +code statically typed. + +Looking for ways to make your assertions simpler - and higher level - will +often nudge you towards making the entire system simpler and more robust. + +Good code is code where you can poke around and see what it's doing - +introspection. We can't debug anything if we can't see what's going on. + +Whenever we're debugging, and the solution isn't immediately obvious, if the +issue is that we don't know where the issue is because we can't see what's +going on - fix that first. + +We have the tools to make anything visible at runtime, efficiently - RCU and +percpu data structures among them. Don't let things stay hidden. + +The most important tool for introspection is the humble pretty printer - in +bcachefs, this means `*_to_text()` functions, which output to printbufs. + +Pretty printers are wonderful, because they compose and you can use them +everywhere. Having functions to print whatever object you're working with will +make your error messages much easier to write (therefore they will actually +exist) and much more informative. And they can be used from sysfs/debugfs, as +well as tracepoints. + +Error messages should, whenever possible, tell you everything you need to debug +the issue. It's worth putting effort into them. + +Tracepoints shouldn't be the first thing you reach for. They're an important +tool, but always look for more immediate ways to make things visible. When we +have to rely on tracing, we have to know which tracepoints we're looking for, +and then we have to run the troublesome workload, and then we have to sift +through logs. This is a lot of steps to go through when a user is hitting +something, and if it's intermittent it may not even be possible. + +The humble counter is an incredibly useful tool. They're cheap and simple to +use, and many complicated internal operations with lots of things that can +behave weirdly (anything involving memory reclaim, for example) become +shockingly easy to debug once you have counters on every distinct codepath. + +Persistent counters are even better. + +When debugging, try to get the most out of every bug you come across; don't +rush to fix the initial issue. Look for things that will make related bugs +easier the next time around - introspection, new assertions, better error +messages, new debug tools, and do those first. Look for ways to make the system +better behaved; often one bug will uncover several other bugs through +downstream effects. + +Fix all that first, and then the original bug first - even if that means +keeping a user waiting. They'll thank you in the long run, and when they +understand what you're doing you'll be amazed at how patient they're happy to +be. Users like to help - otherwise they wouldn't be reporting the bug in the +first place. + +Talk to your users. Don't isolate yourself. + +Users notice all sorts of interesting things, and by just talking to them and +interacting with them you can benefit from their experience. + +Spend time doing support and helpdesk stuff. Don't just write code - code isn't +finished until it's being used trouble free. + +This will also motivate you to make your debugging tools as good as possible, +and perhaps even your documentation, too. Like anything else in life, the more +time you spend at it the better you'll get, and you the developer are the +person most able to improve the tools to make debugging quick and easy. + +Be wary of how you take on and commit to big projects. Don't let development +become product-manager focused. Often time an idea is a good one but needs to +wait for its proper time - but you won't know if it's the proper time for an +idea until you start writing code. + +Expect to throw a lot of things away, or leave them half finished for later. +Nobody writes all perfect code that all gets shipped, and you'll be much more +productive in the long run if you notice this early and shift to something +else. The experience gained and lessons learned will be valuable for all the +other work you do. + +But don't be afraid to tackle projects that require significant rework of +existing code. Sometimes these can be the best projects, because they can lead +us to make existing code more general, more flexible, more multipurpose and +perhaps more robust. Just don't hesitate to abandon the idea if it looks like +it's going to make a mess of things. + +Complicated features can often be done as a series of refactorings, with the +final change that actually implements the feature as a quite small patch at the +end. It's wonderful when this happens, especially when those refactorings are +things that improve the codebase in their own right. When that happens there's +much less risk of wasted effort if the feature you were going for doesn't work +out. + +Always strive to work incrementally. Always strive to turn the big projects +into little bite sized projects that can prove their own merits. + +Instead of always tackling those big projects, look for little things that +will be useful, and make the big projects easier. + +The question of what's likely to be useful is where junior developers most +often go astray - doing something because it seems like it'll be useful often +leads to overengineering. Knowing what's useful comes from many years of +experience, or talking with people who have that experience - or from simply +reading lots of code and looking for common patterns and issues. Don't be +afraid to throw things away and do something simpler. + +Talk about your ideas with your fellow developers; often times the best things +come from relaxed conversations where people aren't afraid to say "what if?". + +Don't neglect your tools. + +The most important tools (besides the compiler and our text editor) are the +tools we use for testing. The shortest possible edit/test/debug cycle is +essential for working productively. We learn, gain experience, and discover the +errors in our thinking by running our code and seeing what happens. If your +time is being wasted because your tools are bad or too slow - don't accept it, +fix it. + +Put effort into your documentation, commmit messages, and code comments - but +don't go overboard. A good commit message is wonderful - but if the information +was important enough to go in a commit message, ask yourself if it would be +even better as a code comment. + +A good code comment is wonderful, but even better is the comment that didn't +need to exist because the code was so straightforward as to be obvious; +organized into small clean and tidy modules, with clear and descriptive names +for functions and variable, where every line of code has a clear purpose. diff --git a/Documentation/filesystems/bcachefs/index.rst b/Documentation/filesystems/bcachefs/index.rst index e2bd61ccd96f..7fb8d54e150a 100644 --- a/Documentation/filesystems/bcachefs/index.rst +++ b/Documentation/filesystems/bcachefs/index.rst @@ -8,4 +8,5 @@ bcachefs Documentation :maxdepth: 2 :numbered: + developer-guide errorcodes -- cgit v1.2.3